Ah ha!  One more mystery solved.  Sort of.  They aren’t really saying how they know the secret of Common Grounds, just that they know it.  And most importantly, that they keep it secret.  I’m sure the werewolves would appreciate that, if they knew.  Of course, being both territorial and rather paranoid, the werewolves wouldn’t appreciate non-wolf folk knowing their secrets at all, so it’s probably a good thing this secret society is…well, secret.