It has come up several times in chat that it would be useful to be able to ping a group of people in one go without having to find the id of each individual person.
It would be particularly useful in the Teachers Lounge when a moderator from site A wants to contact someone from site B to discuss a potential question migration.
It would also be useful there to contact the community coordinators, CHAOS team or even the developers (though this could be abused).
In the site specific rooms users could ping that site's moderators.
All users in the group would get the message in their inbox. A nice to have feature would be to remove it once the ping had been responded to by someone. If this can't be implemented then perhaps the ping should disappear after a while under the assumption that someone from the group has replied.
As Adam has pointed out in the comments, this may well need to be rate limited in some way to prevent abuse. That was part of my point about removing the ping when someone from the group replied.