BuddyPress.org

If everyone’s going to be friends with everyone else, I’d suggest just turning the Friends component off since it won’t be doing anything interesting. Dashboard > BuddyPress > Component Setup

As for groups, you have a few options. If you make all groups public, then everyone will have access to everything. And if you hide all of the “my groups” and “join this group” buttons in the theme, people will never know (or care!) whether they’re a part of groups. They’ll just be auto-joined if they choose to post on the forums. A different method is to modify something like this: https://buddypress.org/community/groups/auto-join-groups/