710 ILCS 15/4

By consent of all parties to an arbitration proceeding, a person, corporation, or entity not a signatory to the agreement may be invited to participate in and be bound by the agreement, or may be accepted into the agreement upon an offer to participate and be bound. If such invitation or acceptance is made pursuant to consent of the arbitration parties, no signatory may refuse to arbitrate because of the participation of such additional party. An additional participant shall execute a written statement to be bound by the arbitration proceedings and agreement or shall sign the agreement, and shall then be treated as a party.