I have been to a party like you describe, but usually LSers want to stay low key, so an add on BP is very odd, and I would guess is going to turn into a full blown (no pun intended) sausage fest with very little participation by the ladies unless you get lucky enough where one wants to pull a train or be gangbanged....at the best, it's gonna be a roll of the dice.    
 

I will wager that the massive swingers party is actually a commercial enterprise. They seem to have recruited some ladies and are most likely renting a hotel room for the 'party'. It's not a swingers gathering in the true sense of the word, more like an orgy with a paid admission.

there are several legit websites that advertise for the lifestyle.. i am longtime swinger of 15 years...each sites is great for the specific area you live in.. there are hoteltakeover parties,clubs,social events, resorts, cruises, etc.

I'm involved in the kink/BDSM community and a lot of the events coincide with swinger events...The only thing I ever see that early in the week is rope practice/munches/sloshes...

Besides...single guys are probably not going to get much play at a swingers party.  Try your local kink community where events are more single male friendly.