Here is the screen in S2Member on the General Options section that I think is controlling this form:
Open Registration / Free Subscribers (optional)
s2Member supports Free Subscribers (at Level #0), along with four Primary Levels [1-4] of paid Membership. If you want your visitors to be capable of registering absolutely free, you will want to “allow” Open Registration. Whenever a visitor registers without paying, they’ll automatically become a Free Subscriber, at Level #0.
Allow Open Registration? (Free Subscribers)
Yes (allow Open Registration; Free Subscribers at Level #0)
If you set this to Yes, you’re unlocking /wp-login.php?action=register. When a visitor registers without paying, they’ll automatically become a Free Subscriber, at Level #0. The s2Member software reserves Level #0; to be used ONLY for Free Subscribers. All other Membership Levels [1-4] require payment.
BuddyPress: BuddyPress will use its own Registration Form here.
s2Member integrates with BuddyPress, and the above setting will control Open Regisration for BuddyPress too.
