I'm doing withered army on 5 toons and the cache on 7 toons (given that there are actually 4 Nightfallen WQs... Most of the time there is only 3) and have yet to get the invitation.
To put it into perspective, I have 5/7 of my 110s with a legendary (one is wrong spec because Blizzard did a dumb and didn't make a separate legendary loot spec) but haven't looted an invitation. It's not even meant to be that rare...