User group addresses

[kitplummer]

Customer is interested in being able to @ message a group. And that the group collection should be dynamic. For example the group could change based on who is in duty, or assigned to a mission, task or event.

Ref Slacks use of @here and @channel

[ErikEverson]

@here can be solved by knowing who is subscribed to the channel. We would then add that message to the mentions list for all users that are subscribed to the room

Other user groups will likely have to be solved with some other mechanism