So you just need something to keep a list of members that get charged every month?
What are you offering them in exchange for membership? Downloadable files, videos, product that gets physically shipped, or? Or are you taking care of fulfilment on a totally separate system, and you need this system for billing only?
Do you need a referral / affiliate program? What type, if any fees (ie. shipping costs / taxes) need to be supported? Is there different levels of membership, or just one? Do you need multi-currency support?
Again, more details needed...