To enable Group Messaging you need to select the check box on the Group's details and also enter an identifier in field next to the address (to create a group email address).
e.g. in the example below, the Group Message address will be:

To enable Group Messaging you need to select the check box on the Group's details and also enter an identifier in field next to the address (to create a group email address).
e.g. in the example below, the Group Message address will be: