I have found that i can use Gravity Forms (or any other WP form plugin) to create a user then redirect them to the Memberships page, when they click an item they are then made a member.
So the process currently is:
Signup link -> Register -> Select Membership -> Done
The client has requested that the process be
Select Membership -> Register -> Done
Is there anything i can do with field population in the Register form or is the current process vital to the functionality of the plugin?