1. A method of a service capability server/application server (SCS/AS) in a wireless network, the method comprising:transmitting, to a service capability exposure function (SCEF), a first request message to request an allocation of a temporary mobile group identifier (TMGI) in a case that there is no allocated TMGI for a group message delivery, the first request message including an external group identifier and an SCS/AS identifier;
receiving, from the SCEF, a first response message including the TMGI in a case that the SCS/AS is authorized to request the allocation of the TMGI;
transmitting, to a user equipment (UE), an application-level signaling including the TMGI; and transmitting, to the SCEF, a request message including the TMGI and the external group identifier for requesting the group message delivery,
wherein the SCEF checks whether the SCS/AS is authorized to request the allocation of the TMGI in a case that the first request message is received from the SCS/AS.