If you have a member and can't find them, try looking in the Vines OS menu section called CRM (or Customer Loyalty). This is where customers who are not members, but guest buyers or visitors, are stored.
If you are trying to add a member using an email that it will not accept, and you get a message that the email is already being used, go to: CRM > Customers. Do a Search for the email. Open the Customer record, and add them to the Club.
Make sure to add the credit card details.