I wouldn't use 'enrol' at all in that context. 'Enrol' is more for education e.g. 'I enrolled on a course". Two options are 'take out' membership (we also use 'take out' for subscriptions, insurance policies and similar) or 'sign up for' membership. Another natural phrasal verb in this context, especially if you need to make a decision between standard and premium membership, for example, is 'I'll go for the premium membership' meaning 'I'll choose' ( as in 'I'll go for the fish' when you decide what to choose from a menu).

Here, I would use "for" actually. "I will enroll for Premium membership. This is the "level of membership". If you were enrolling "at" it would normally be a place (ex: enroll at a gym, enroll at a university). If you are enrolling "in" it could be a enrolling in a program. Enroll to, I cannot think of any expressions using this preposition. Ciao!

In