Rule 24.Intervention.

Rule 24 lets someone who is not already a party ask to join a pending lawsuit. Subdivision (a) covers intervention of right: on a timely motion, the court must let the applicant in if a statute grants an unconditional right to intervene, or if the applicant claims an interest in the subject of the action that could, as a practical matter, be impaired or impeded by the case going forward without them, unless the parties already in the case adequately represent that interest.

Subdivision (b) covers permissive intervention, which rests in the court's discretion rather than being guaranteed. A court may allow someone to intervene if a statute gives a conditional right to do so, or if the applicant's claim or defense shares a common question of law or fact with the main action; a state officer or agency can seek the same kind of permissive intervention when a party's claim or defense rests on a statute or regulation that officer or agency administers. Either way, the underlying motion must be timely, and for permissive intervention the rule also directs the court to consider whether allowing the intervention will unduly delay or prejudice the rights of the parties already in the case.

Subdivision (c) sets the mechanics: anyone moving to intervene must serve the motion on the existing parties and attach a proposed pleading spelling out the claim or defense the intervention seeks to advance. Once the court grants the motion, the intervenor must file and serve that pleading within 10 days unless the court sets a different deadline, and any party who must respond to it has 20 days to do so.