The concept I think is most relevant is that used by NA for their Western Service Learning Days (WSLD). To NA, Western means the western half of North America. They invite Mexico - like we invite the Baja California Region. They also invite Canada - and we have extended invitations to the British Columbia Region.
The selection should be through attraction. Any region that wants to participate is welcome to help do the work. Any member that wants to participate is welcome to attend the event.