A method of operating an application server that is external to a core network and configured to arbitrate a group communication session for transmission of session media to a multicast group being served by the core network, comprising: determining, by the application server that is external to the core network, a first support state for the group communication session based on one or more UE registrations to the group communication session and/or one or more UE de-registrations from the group communication session, the first support state defining a first set of sectors belonging to a multicast/broadcast single frequency network, MBSFN, area for transmission of the session media for the group communication session via Internet Protocol, IP, multicast; delivering, by the application server that is external to the core network, the session media for the group communication session to the first set of sectors via IP multicast in accordance with the first support state; transitioning, by the application server that is external to the core network, the group communication session from the first support state to a second support state based on one or more additional UE registrations to the group communication session and/or one or more additional UE de-registrations from the group communication session, the second support state defining a second set of sectors belonging to the MBSFN area for transmission of the session media for the group communication session via IP multicast, wherein the first set of sectors is different from the second set of sectors; and delivering, by the application server that is external to the core network, after the transition, the session media for the group communication session to the second set of sectors via IP multicast in accordance with the second support state. determining, by the application server that is external to the core network, a first support state for the group communication session based on one or more UE registrations to the group communication session and/or one or more UE de-registrations from the group communication session, the first support state defining a first set of sectors belonging to a multicast/broadcast single frequency network, MBSFN, area for transmission of the session media for the group communication session via Internet Protocol, IP, multicast; delivering, by the application server that is external to the core network, the session media for the group communication session to the first set of sectors via IP multicast in accordance with the first support state; transitioning, by the application server that is external to the core network, the group communication session from the first support state to a second support state based on one or more additional UE registrations to the group communication session and/or one or more additional UE de-registrations from the group communication session, the second support state defining a second set of sectors belonging to the MBSFN area for transmission of the session media for the group communication session via IP multicast, wherein the first set of sectors is different from the second set of sectors; and delivering, by the application server that is external to the core network, after the transition, the session media for the group communication session to the second set of sectors via IP multicast in accordance with the second support state.