Like everyone else, I echo their caution. But as I've gotten older, my views have mellowed a bit and I'm not quite as cut and dried about it. Why can't we have good friends with benefits? I think it's the ultimate solution, really. The trick is to find the right kind of friends, and to leave room for growth.
It sounds like you've had some fun already, which is great. I'd suggest that you treat every new encounter as a one-off, realizing that anyone can change their mind about how okay they are with it at any time. As cplnuswing said, leave room for a graceful exit. Sex makes people behave funny; it's a very intimate part of their lives and their relationship together. Swinging often brings about enormous growth in the relationship and in one's own understanding of one's self. Because of this, their interest in swinging will change over time, and you may or may not be a part of it. If you choose to go forward with playing with them, just recognize this and make allowances for it.
I'd also suggest not being too coy with them. If they seem a little embarrassed about your alcohol-fueled party games, I'd try to get a conversation going about it, letting them know that you had a great time, and that you don't regret it at all. If you can find a way to get a dialogue going with them, you can maybe give them the opportunity to freely ask questions and talk about the way you two look at things (especially how you view playing with friends), and you can ask them - as Alura suggested - what they think about it all. Because you're curious. If the question comes up, does everyone want to play, yes or no?, then maybe gentle things a little by saying you're interested in seeing where things go, but that you really value your friendship with them, so they need to be straight with you if they ever feel uncomfortable at any time.