Book clubs and other discussion-focused groups that are open to new attendees often put up a "flag" so the new people can find the group in a public place. It's

Book clubs and other discussion-focused groups that are open to new attendees often put up a "flag" so the new people can find the group in a public place. It's just not a literal flag, but some other kind of sign. If the group's activity is easily visible and distinct from what everyone else is doing, it's not necessary.