One way to way too seed activities would be to let professional organizers advertise on the platform (also solves monetization). E.g. a local sports club could offer a trial training session for small groups through the platform.

Trying something new is usually slightly uncomfortable when you're the only newbie (and everyone else has to accommodate you); and in the clubs I've been a member of, those who stuck around for longer were almost always already friends with another member.

Having whole groups sign up at once gives the club a better chance at retaining them and should also provide a fun experience for everyone even if they don't decide to stay.