BuddyPress.org

Try this link to change it to the forums instead of the activity: https://buddypress.org/community/groups/how-to-and-troubleshooting/forum/topic/changing-group-tab-display-defaults#post-55449

I think you can turn on or off the reply button for forums in the BP admin, but they aren’t completely integrated that way. A reply in the activity won’t appear in the forum. So, that’s why it’s disabled by default (I think).