Hi @CeceRankin,
Unfortunately, I believe the answer to this one is "no." The main board, even in group discussions, still exists, but is only really accessible to TAs, teachers, and students who have not been assigned a group. If you want to add reply content so everyone can see it, you'd need to reply individually to each group discussion board.
Alternately, you could modify your discussion prompt to include the info you're trying to convey if that would work (and if you have permission to do so). If you edit the discussion prompt, you would just do that once and it would apply to all of the group discussions.
-Chris
