Keep it simple!!!
Your membership site is not like a bank account - let users choose their login and password and allow them to keep those simple if thye want to.
Anything that says: your password must contain at least one digit and a number.... might be useful in banks and payments or sensitive info but in your membership site, you don't store credit card and sensitive information.
So, some security is good (password) too much of it is NOT NEEDED.
Don't make that mistake!
Again keep it simple!