BuddyPress.org

In wp-includeswpmu-functions.php change this:

add_user_to_blog(‘1’, $user_id, ‘subscriber’);

to this:

add_user_to_blog(‘1’, $user_id, ‘author’);

That should take care of all future signups, however existing users will be unchanged. If you don’t have too many users yet you can simply manually change them to authors, however, if you have a lot of signups check out this thread for a way to mass add users as authors.