I am unaware about how exactly openfire broadcast plugin works, but if you globally disable offline messages inside openfire, does broadcast plugin still sends out offline messages? If so, in that case broadcast plugin probably doesn't follow the global configs.
What kind of groups are these? Are these same MUC specification groups? Also what kind of membership does room members have? Ideally if you send out a message into a MUC room, it is being received only by the current members of the group. You can configure no history archive policy for MUC groups too.
Yes, you can pull list of members in a group using JAXL. When you enter into a MUC room, server will send you presence notification for every member in the group. Kindly checkout
muc_log_bot.php and extend over it as required.