How does the Membership plugin work?

Hello WPMU Dev Community, i am just using the Membership plugin on a website of mine and am having a bit of trouble understanding how it works.

Does a member register for a plan after they have already created an account or when they create there account, how does the registration process work?

I am using WordPress Multisite, BuddyPress and the Membership system setup on Dripped Content Subscriptions. I have already been through half the docs but just need some help.

Cheers,