BuddyPress.org

@nahummadrid – Thanks! That clears up things!
You should preferably add the users to the blog.

Try a plugin like this:
https://wordpress.org/extend/plugins/wpmu-default-user-role/

Or, customize this per blog with:
add_user_to_blog( $key, $user_id, $value );

https://codex.wordpress.org/WPMU_Functions/add_user_to_blog