At least one embodiment takes the form of a process carried out by a first radio-network switching system (RNSS). The first RNSS maintains a talkgroup of mobile radios that includes a first mobile radio. The first RNSS receives an add-mobile-radio request from the first mobile radio and responsively sends an add-mobile-radio response to the first mobile radio. The add-mobile-radio response includes an add-mobile-radio authentication token. The first RNSS receives, from a second mobile radio, an add-mobile-radio authentication message that identifies the second mobile radio and that includes the add-mobile-radio authentication token. In response to receiving the add-mobile-radio authentication message from the second mobile radio, the first RNSS adds the identified second mobile radio to the talkgroup.