The blue post on the following page says you can only get them from the Black Market (which I think is a bad idea):
But the patch notes on live do say:
"Invitations can be found on the black market auction house, by invitation from somebody within the guild, and occasionally as drops from certain Horde and Alliance NPCs."
The patch notes match the ones from the PTR from before the blue post above was made. So it's possible I guess that they simply weren't updated.
I've killed a total of 5 rares now and haven't seen an invitation.
Would love confirmation one way or the other.
I'm in the same boat, I'll let you know if I get one. Got 250 honor items, and the Resilience Trinket so far.