at the top of the body of the forum, it will tell you who started it, and what kind of forum it is: Invite Only; Moderated; or Public. you have to write to the forum opener to join many of them, requesting admission. some people come to PC less often than an interested party might like, so you might have to wait a while. there is also a link to the original member group that made the forum, if you want to send an Invitation Request. sometimes those are hard to notice. better to write to the group founder.

glad you are finding things of interest~ best wishes~
Gus