It takes some work and certainly the right tools to set up a members-only website but the rewards (monetary as well as personal satisfaction) can be huge. Whatever you are passionately interested in or very adept at can be the topic of a members-only website today.
One of the first types of membership sties on the Internet was the online dating site. In the beginning, they were mostly populated with nerds, weirdoes and perverts but as the Internet matured the sites became popular among people from all walks of life. They have even progressed so far as to offer pre-screening of members. Most porn sites are membership only sites.
There are membership sites today that cover just about every area of human interests. Some offer informational or teaching material that is only able to be accessed by site members. Some membership sites are able to be viewed by non members but non-members are not allowed to participate in the activities or post to the message boards on the site. Sites that provide information to find work-at-home jobs, for example, can see the listing of jobs but cannot apply for the jobs unless they are members of the membership site.
There are membership sites that provide instruction is such things as how to play a guitar. The lessons are accessible only to members of the site, although those who are not members can view what subjects are being taught in the lessons.
Membership sites usually accept payment for membership fees by credit card or by personal check.