The present document specifies the stage 2 description of the Supplementary Service Include Call (SS-IC) for the Terrestrial Trunked Radio (TETRA). SS-IC enables a served user, while already participating in an active call (original call) to include new participant(s) into a group call together with the participant(s) in the original call. Man-Machine Interface (MMI) and charging principles are outside of the scope of the present document.