735 ILCS 5/2-404
All persons may join in one action as plaintiffs, in whom any right to relief in respect of or arising out of the same transaction or series of transactions is alleged to exist, whether jointly, severally or in the alternative, whenever if those persons had brought separate actions any common question of law or fact would arise. If upon the application of any party it shall appear that joinder may embarrass or delay the trial of the action, the court may order separate trials or enter any other order that may be expedient. Judgment may be entered for any one or more of the plaintiffs who may be found to be entitled to relief, for the relief to which he or she or they may be entitled.
If any one who is a necessary plaintiff, counterclaimant or third-party plaintiff declines to join, he or she may be made a defendant, cross defendant or third-party defendant, as the case may be, the reason therefor being stated in the complaint, counterclaim or third-party complaint.
735 ILCS 5/2-404