Given that many of the Ad Astra members who post their stories on the archive often do so after having posted it here on the trekbbs fanfiction board and are regular members on both sites I rather imagine that there is no stealing of members only putting the word out. I serve to be corrected by the board administration about their stand but it was never a problem in the past when Ad Astra advertised writing challenges etc within this forum. Many such stories that were then posted and shared on this forum promoting its activity.
Further, many Ad Astra members are prolific writers here in the fanfiction forum. You can do a quick check and see just by the first page of the TrekBBS fanfiction board that 5 of the frontpage authors are regular members on Ad Astra. There are countless others that are members and visit both sites and activity in one begets activity in the other.