I've wanted this feature too, but more to communicate to a small group of gamer friends. The problem is that while you can send to multiple people, they see it as just from you. So they can reply just to you. So really targeted group communication doesn't work via Geekmail, I just use Facebook for now but would prefer BGG as I kind of avoid FB these days. Slightly different issue than the OP obviously.
The times when a group of us tries to communicate by geekmail, we put a list of all recipients
UserA, UserB, UserC
so that people know who got the geekmail, and so that people can copy and paste that row to make sending to everyone easier.