Until recently, new members for a membership site had to be added by an admin. (The membership feature was originally designed to support private membership organizations, in which the membership administrator needs complete control over who is registered.)

You can now provide visitor self-registration forms, so your site visitors can sign up from the site.

Note that this is intended for sites where registration is either free, or for which payment is handled separately; processing of registrations requiring payment remains on the future feature list.