A half-duplex radio communication method, a program and a system enabling multicasting, for example, in an emergency for radio communication terminals belonging to different carrier systems if they meet designated condition while saving an operator the bother of inputting destinations. After accepting an application for emergency multicasting, a CUG server requests a presence server to search for data of radio communication terminals meeting the positional condition designated by a sender and receives the data. In this operation, the CUG server searches a present state table in the presence server, picks up user information of the radio communication terminals as the destinations of the emergency multicasts, based on the positional information. Then, a CUG is formed based on the picked up user information and registered as the CUG for the emergency multicasting.