A motion for joinder is a written request asking a judge to add a party or a claim to a lawsuit that is already underway. To win one, you have to point to the specific Federal Rule of Civil Procedure that authorizes what you’re asking for, show the facts fit that rule, prove the court still has jurisdiction over everyone and everything involved, and file before the scheduling order’s deadline closes the window. Most denials trace back to one of those four pieces, and the jurisdictional piece is where otherwise strong motions quietly fall apart.
What the Motion Has to Prove
Every motion for joinder, regardless of which rule it invokes, has to accomplish three things on the page: explain the factual connection between the new party or claim and the existing case, establish the legal basis under a specific rule, and demonstrate that the court has jurisdiction over the expanded lawsuit. Courts have little patience for motions that recite the standard without showing how the facts meet it.
The burden sits on the party asking for joinder. Opponents routinely argue undue delay, prejudice to existing parties, the absence of a genuine common question of law or fact, jurisdictional defects, or that the proposed new claims would be futile. A motion that doesn’t anticipate and answer those objections tends to lose.
Picking the Right Rule
Joinder isn’t one rule, it’s a family of them. The right one depends on who or what you’re trying to add and why.
Rule 20: Permissive Joinder of Parties
Rule 20 is the most common route. Plaintiffs may join together, or defendants may be joined, when two conditions are both met: the claims arise from the same transaction or series of connected transactions, and at least one question of law or fact is common to all the joined parties.1Legal Information Institute. Federal Rules of Civil Procedure Rule 20 – Permissive Joinder of Parties Both prongs matter. Shared facts alone aren’t enough if the claims come from unrelated events, and a common transaction isn’t enough if the legal questions are entirely different.
Even when the test is satisfied, the court has discretion to limit joinder. Rule 20(b) authorizes protective measures, including separate trials, to shield a party from embarrassment, delay, or expense caused by being lumped in with parties who assert no claims against them and against whom they assert no claims.1Legal Information Institute. Federal Rules of Civil Procedure Rule 20 – Permissive Joinder of Parties
Rule 19: Required Joinder of Parties
Rule 19 is less flexible and more consequential. A party must be joined when their absence would prevent the court from granting complete relief to the existing parties, or would impair the absent party’s ability to protect an interest tied to the lawsuit.2Legal Information Institute. Federal Rules of Civil Procedure Rule 19 – Required Joinder of Parties It also applies when proceeding without the absent party would expose an existing party to double or inconsistent obligations. A motion under Rule 19 should show why the absent party’s interests are so intertwined with the litigation that the case can’t be fairly decided without them.
Sometimes a required party can’t be brought in, usually because joining them would destroy the court’s jurisdiction or because they’re beyond the court’s reach. When that happens, Rule 19(b) requires the judge to weigh four factors to decide whether the case should go forward or be dismissed:
- How much prejudice a judgment without the missing party would cause.
- Whether protective measures could reduce that prejudice.
- Whether a judgment rendered in the party’s absence would be adequate.
- Whether the plaintiff would have an adequate remedy if the case were dismissed.
If those factors tip against proceeding, the court dismisses. The missing party is then called “indispensable.”2Legal Information Institute. Federal Rules of Civil Procedure Rule 19 – Required Joinder of Parties A plaintiff who has invested months in a case can lose it because a necessary party was never brought in.
Rule 18: Joinder of Claims
Rule 18 is strikingly broad. A party already asserting a claim, counterclaim, crossclaim, or third-party claim may join as many additional claims as they have against the opposing party, related or not.3Legal Information Institute. Federal Rules of Civil Procedure Rule 18 – Joinder of Claims A plaintiff suing a business for breach of contract can tack on a completely unrelated personal injury claim against that same business in the same lawsuit. The practical limit is jurisdiction: every joined claim must independently fall within the court’s authority or qualify for supplemental jurisdiction.
Rule 14: Impleader
Impleader lets a defendant bring in a non-party who may be liable for all or part of the plaintiff’s claim against the defendant. The classic use: a general contractor sued for a construction defect brings in the subcontractor who did the faulty work. A defendant can file a third-party complaint within 14 days of serving the original answer without the court’s permission. After that, a motion for leave is required.4Legal Information Institute. Federal Rules of Civil Procedure Rule 14 – Third-Party Practice
Impleader is narrower than it looks. The third-party claim has to be derivative, meaning the third party’s liability depends on the defendant’s liability to the plaintiff. You can’t use Rule 14 to pull in someone over a separate dispute.
Rule 13: Counterclaims and Crossclaims
A defendant adding their own claims doesn’t always need a joinder motion. A compulsory counterclaim arises from the same transaction or occurrence as the plaintiff’s claim, and the defendant must raise it in the answer or lose it. A permissive counterclaim can be raised but doesn’t have to be. A crossclaim against a co-party is allowed only if it arises from the same transaction or occurrence as the original action, or relates to the property at the center of the dispute.5Legal Information Institute. Federal Rules of Civil Procedure Rule 13 – Counterclaim and Crossclaim
Timing: The Deadline That Kills Most Late Motions
Timing is the single most underestimated piece of joinder. In virtually every federal case, the judge issues a scheduling order early in the litigation that sets a firm deadline for adding parties and amending pleadings.6Legal Information Institute. Federal Rules of Civil Procedure Rule 16 – Pretrial Conferences; Scheduling; Management Miss it, and the standard jumps.
Before the Deadline
Adding a party or claim before the scheduling deadline usually means amending the complaint. Under Rule 15(a), a party can amend once as a matter of course within 21 days of serving the pleading, or within 21 days after a responsive pleading or Rule 12 motion is served, whichever comes first.7Legal Information Institute. Federal Rules of Civil Procedure Rule 15 – Amended and Supplemental Pleadings After that early window, amendment requires the opposing party’s written consent or the court’s leave. Courts are instructed to freely grant leave when justice requires it, so the bar at this stage is relatively low.
After the Deadline
Once the scheduling order deadline passes, a party must first satisfy the “good cause” standard of Rule 16(b)(4) before the court will even consider the more permissive Rule 15 standard.6Legal Information Institute. Federal Rules of Civil Procedure Rule 16 – Pretrial Conferences; Scheduling; Management Good cause turns on diligence. A party who knew about the potential new claim or party for months but waited will almost certainly be denied. The question is whether you acted promptly once you had reason to seek joinder, not whether the joinder itself has merit.
Statute of Limitations and Relation Back
Adding a new party raises a statute of limitations problem: the filing deadline for suing that party may have already run. Rule 15(c)(1) addresses this through “relation back,” which treats the amended pleading as filed on the same date as the original complaint. For a newly added party, relation back applies only when all of the following are true:
- The new claim arises from the same conduct or transaction described in the original complaint.
- The party being added received notice of the lawsuit within the time allowed for serving the summons and complaint, so they won’t be prejudiced in mounting a defense.
- That party knew or should have known the action would have been brought against them but for a mistake about the correct party’s identity.
The last requirement is where most relation-back arguments fail. Courts distinguish between a genuine mistake about identity (naming “ABC Corp” when you meant “ABC LLC”) and a deliberate choice not to sue someone. Changing your mind about who to name typically doesn’t qualify as a “mistake.”7Legal Information Institute. Federal Rules of Civil Procedure Rule 15 – Amended and Supplemental Pleadings
The Jurisdiction Trap in Diversity Cases
Joinder doesn’t override the court’s jurisdictional limits. Every added party and every joined claim must fall within the court’s authority. In federal court, that means a federal question, diversity of citizenship with more than $75,000 in controversy, or supplemental jurisdiction.
Supplemental jurisdiction under 28 U.S.C. ยง 1367 allows federal courts to hear claims closely related to claims already before the court, even if those related claims couldn’t stand alone in federal court. But Congress carved out important limits for diversity cases. When jurisdiction rests solely on diversity, supplemental jurisdiction does not extend to claims by plaintiffs against parties brought in under Rules 14, 19, 20, or 24, or to claims by persons seeking to join as plaintiffs under Rule 19 or intervene under Rule 24, when doing so would undermine the diversity requirement.8Office of the Law Revision Counsel. 28 USC 1367 – Supplemental Jurisdiction
In plain terms: if your case is in federal court only because of diversity, you can’t use joinder to bring in a new plaintiff from the same state as the defendant and lean on supplemental jurisdiction to hold the case together. This catches people off guard, and failing to address it is one of the fastest ways to get a joinder motion denied. Courts won’t do the jurisdictional analysis for you.
Drafting the Motion
A well-drafted motion follows the proof requirements directly. For permissive party joinder under Rule 20, identify the specific transactions or events that connect the new party to the existing lawsuit and spell out what legal or factual questions the new party shares with the current parties. For required joinder under Rule 19, show why the absent party’s interests are so intertwined with the litigation that the case can’t be fairly decided without them. For impleader, show that the third party’s liability flows from your own.
Devote real space to jurisdiction. If adding the new party or claim changes the diversity calculus or sits outside the court’s supplemental jurisdiction, address that head-on. If the scheduling order’s deadline for amendments has passed, build a diligence record and argue good cause under Rule 16(b)(4) before you even reach the Rule 15 standard.
Most courts expect the proposed amended pleading as an attachment, whether that’s an amended complaint adding a new party or a third-party complaint for impleader. This lets the judge and opposing counsel see exactly what the case would look like after joinder, rather than evaluating the request in the abstract. The proposed pleading should be complete and ready to file.
What Happens After You File
Filing goes through the court’s electronic filing system in most federal courts, and it triggers a service obligation: every existing party must receive a copy of the motion and the proposed pleading. Opposing parties then have a window to file a written opposition. The Federal Rules don’t set a universal deadline for responding to non-dispositive motions; most federal district courts set a 14-day response period through their local rules, though some use different timelines or let the judge set one case by case.
A motion for joinder doesn’t take effect just because it was filed. The judge reviews the motion, any oppositions, and often hears oral argument before ruling. Once the judge grants the motion and issues a formal order, the newly added party or claim becomes part of the case. If a new defendant is added, they must be properly served with a summons and the amended complaint, and they get their own deadline to respond. Until that order issues, the joinder is not effective, no matter how strong the motion looks on paper.
If You Got Joinder Wrong
Getting joinder wrong is not necessarily fatal. Rule 21 provides that misjoinder of parties is not grounds for dismissing an action. The court can add or drop parties on its own or on motion, at any stage of the case, on whatever terms are just.9Legal Information Institute. Federal Rules of Civil Procedure Rule 21 – Misjoinder and Nonjoinder of Parties The court can also sever a claim against a party and send it to a separate proceeding.
This safety valve matters in practice. If a defendant argues that a plaintiff improperly joined parties to defeat diversity jurisdiction, a common tactic in removal cases, the remedy is to drop the misjoined party, not to throw out the entire case. Don’t treat Rule 21 as a fallback plan, though. Judges notice when parties play loose with joinder rules and then rely on the court to clean up afterward.