BuddyPress.org

Problem:
“* Note: The s2Member plugin has control over four options on this page.
Dashboard Blog = 1 / Main Site, Default Role = subscriber, Allow Open Registration = none, and Add New Users = 1.
In your Dashboard ( on the Main Site ), see: s2Member -> Multisite ( Config ).”

Set s2Member -> Multisite(config) -> Multisite Registration ( Main Site Configuration ) -> Yes allow open registration