You should add it to user.databases via the Server menu. Candidates for membership are controlled by LDAPscope, so there is no Join option.
People know their usernames and passwords already.
Change member.login to call LDAP.login which is a heavily modified version of the manila script. I thought of member.signup as a real challenge, because it's so complex. Then I got to thinking. If the user table creation is part of login, you don't need signup at all. Note Mainresponder stores passwords in cookies.