WP eMember uses one registration form which is capable of detecting which membership level a user is permitted to signup for. As an example, if a user pays for the "Gold" level then after the payment it will only let this user signup for the gold membership level. If a user directly goes to the registration page without paying then it will only let this user signup for a free membership given you allow free membership on your site. A user cannot signup for a paid/premium membership without making a payment first (Just visiting the registration page without making a payment won't help). After the payment the plugin will send a unique registration link (valid for one user registration) to this users email address. The user will be able to register for a premium account by following that unique link.
If you need to create a registration form for a particular membership level for whatever reason you can do that too with eMember. Registration forms created using this method allows the user to directly register for the specified membership level without making any payment:
http://www.tipsandtricks-hq.com/wordpress-membership/create-a-registration-form-for-a-particular-membership-level-429
Make sure you also read the "Registration Flow" documentation which have more details on the registration flow:
http://www.tipsandtricks-hq.com/wordpress-membership/a-typical-member-registration-flow-96