I never had to pay for the site but honestly even if they charge, so what? You can spend so much wasted time on sites like this trying to weed out fakes and flakes. People that are serious about meeting aren't going to bawk at a few dollars for a reliable way to screen potential partners. When my husband and I were still swinging, a single evening represented hundreds of dollars invested, from a hotel, travel, overnight babysitter, etc. Even so we were stood up more than once.
The most reliable way we ever used for meeting BBC was through a swinger's magazine, people that take the time to write a physical letter and have the patience to wait for a response are serious about actually meeting.