Make all group members Messages buddies

Use the Server app to automatically make all members of a group Messages buddies.

When group members open Messages, the server is included as a Messages server, and group members are included in the list.

Select Groups in the Server app sidebar.
Double-click a group.
To set this option for several groups, hold down the Command key while selecting groups, then click the Action pop-up menu , then choose Edit Group.
Select “Make group members Messages buddies.”