I've never done it, and don't really intend to change that. One-on-one is perfect for me. Plus, I'm rather picky about who I shag, and have high demands for personal chemistry, so the odds of me being attracted to everyone involved equally (or at all) aren't promising. I don't even get turned on by group sex porn and find it rapidly boring/difficult to focus on whom I like best when I come across such scenes.
I've been invited by a few couples to play around and have politely declined. Maybe someday I might be tempted by the right couple, though - it would help if it were a couple that hadn't really tried it before and were personally interested in me, rather than trolling Manhunt for anybody up for it. And again if I am equally into both.