Tenn. Code Ann. § 20-1-114: Addition of parties

(a) At any time before trial, new plaintiffs or defendants may be added to the suit by the plaintiff, upon supplemental process taken out and served or appearance entered.

(b) If added at the appearance term, it may be done without costs; if at any subsequent term, on such terms as to costs and conditions as the court may prescribe, so as to prevent delay.