We are looking at the same problem, and the Alumni Patrol as mentioned earlier has the problem that you not only can’t use Everyone, but you have to select all of the other patrols. It dramatically complicates the selection of who is invited.
Another option is using an external list like Google Groups, but as it looks like you have to be on TroopTrack to RSVP to an event, you would need some other mechanism to handle that for the Alumni.