BuddyPress.org

Add users manually is easy.
Go to .../wp-admin/users.php on your website and click on the button at the top that says ‘Add New‘.

Re add users to group in wp-admin:
Please see this sticky topic.

To narrow down problems, use a default WP theme like 2016.
Deactivate other plugins.
After everything is working, then start using your preferred assets.