As mentioned you'll generally need to bill the users and for that you'll use a
service like CCBill.com. They will provide a very basic script to add and
remove users as they are billed. Their script is pretty low security, though,
and it can be made more secure free or almost free - anyone with some basic
programming experience can do it or we charge $20. The free way of improving
the security has to do with the encryption it uses, which is very weak, and the
weak passwords. These modifications are most easily done ion the beginning,
before your site has any members. This page explains more about stronger
passwords and stronger encryption:
http://bettercgi.com/strongbox/passgen/

Eventually your passwords will get posted on password sites, where potentially
thousands of people will use the same user name and password, costing you
money in hosting bills. You'll also have people using software to try hundreds of
thousands of username / password combinations until they find one that works.
Whenever your budget allows for $100 to protect against those threats you can
get the Strongbox security system to protect your site:
http://bettercgi.com/strongbox/