It’s currently not possible to have a forum without requiring a user to join a group. However, with the following patch, you can:
https://trac.buddypress.org/attachment/ticket/2540/disable_auto_join_constant.patch
Read the details here:
https://trac.buddypress.org/ticket/2540
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Re: second part of your question – read this:
https://codex.buddypress.org/how-to-guides/using-an-existing-bbpress-installation/
Also, your external bbPress posts will not show up in the BuddyPress activity stream without a custom-coded bbPress plugin.
