You have to be invited for those because they are not listed in the directory. You cannot find or search them in Yahoo Groups.
Groups directory categories if desired. Groups also includes so-called adult groups, however, these.
The latest one was an invitation to a Yahoo! Where would a spammer make money by sending out invites to a Yahoo! group? as spam, I'll just ignore it and empty the spam folder (a lot of times I'll do this sight unseen).