I don't think so, not for a group. :( Everything I've seen lists admins, and then users in alphabetical order. You can get people by the order in which they've joined Yammer, but I don't know how you would get them in order of them joining a particular group. Sorry. One thing you can do is run regular reports of group membership and then note who joins each day by comparing one day against another. Would that help your 200-user celebration?