Invision forums right at the moment 105/594 and 2 anon. So given Xen isn't released at yet that's a pretty reasonable comparison, though I for one would like to get the guest numbers up. Please note I'm not saying anyone should go out and spam other sites, no sirrreee bob, that would be wrong and you should be ashamed of yourself for even thinking it.
BTW has anyone brought up Xenforo at slashdot?