I'm sure there are ways to get in touch with multiple people interested in group sex activities. Other forums (or maybe this one), craigslist (there might be stranger things in the personals than this idea), or maybe something like fuckbook.com or other singles websites that are similar.

I have no experience in threesomes, foursomes, or moresomes, so I'm just guessing.

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"BOY: On a hot summer night, would you offer your throat to the wolf with the red roses? GIRL: Will he offer me his mouth? BOY: Yes. GIRL: Will he offer me his teeth? BOY: Yes. GIRL: Will he offer me his jaws? BOY: Yes. GIRL: Will he offer me his hunger? BOY: Yes. GIRL: Again, will he offer me his hunger? BOY: Yes! GIRL: Yes. BOY: On a hot summer night, would you offer your throat to the wolf with the red roses? GIRL: Yes. BOY: I bet you say that to all the boys!" -Meatloaf