Check out our plugin Paid Memberships Pro, which is available in the repository.
http://ww.wp.xz.cn/extend/plugins/paid-memberships-pro/
Members are WordPress users and so have a username and password. There are some ways to get around this, but it’s sticky as it’s pretty important to have a login to… log in for access to the members stuff. Definitely if your do forums. I think you’ll find all WP member plugins create a user account.
Our plugin will play nice with forums plugins, including bbPress/BuddyPress. It can take a few lines of extra code to lock down the forums for members and/or automatically add people to forums on setup. This is straight forward if you have experience coding php and/or tweaking WP plugins.
Good luck.
