Using Memberstack 2.0? This issue has been fixed. Please post a question here if you have a related issue.
The "Undefined" error is usually caused by one of the following issues:
No memberships→ Your site needs at least one membership for users to be able to sign up.
Archived memberships→ Your site needs at least one active membership.
Corrupted cookies → Try clearing your cookies. Cookies can get corrupted if you are making edits in Memberstack while trying to sign up in another window.