Your system administrator can grant you the ability to enable/disable your membership to a group through the Status menu. When your membership to a group is disabled, you do not receive group calls for that group.
The system administrator is able to configure whether you have access to the Membership, State and Fallback options of a group. If you are not configured for any groups, the Group menu is not shown
To do this during a call, press the PHONE key and then press the Status soft key.