The only similar thing that I can comment on is the Lifestyle Gangbang Parties. Now, I know that there has been a recent thread on these, BUT, I comment here based only on my experience of the Sittingbourne evening parties, as these are the only one's I have been too, and based pretty close to Eureka's.
I have been pretty much every month since the start there, and I have only experienced a low ratio party, max has ever been 5:1 in terms of numbers, but in reality, that has never happened in play, as guys are resting etc.
Just recently. they have had couples and single swingling ladies attend, and that creates a perfect gangbang scenerio. I would imagine that if you attended with a partner. you could do as much or as little as you like with other guys. At the last party I attended, a couple were there, but just playing with themselves, but enjoyed being watched. It really is each to their own. The single swinging lady certainly enjoyed herself!
Just wanted to add that I am in no way connected to them at all before all the 'Fanboy' etc comments start.
The club that they use, DV8, also hold regular swinging parties that would also be of interest. They hold a regular Cougars and Cubs night, and that is one that I am looking to attend soon.