BuddyPress.org

Welcome Pack should allow you to automatically join users to groups the site admin defines when their account is activated.

Be sure to read the following as well when using Welcome Pack to force users to groups:
https://buddypress.org/community/groups/how-to-and-troubleshooting/forum/topic/standard-group-for-new-users/#post-69096

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Besides, if you attempt to post to a forum topic that you’re not a member of, you will automatically join that group when you reply to that forum thread.