It has nothing to do with number of participants at all. It's merely a coded word for something that has entirely too many words already.
Seven is useful in many situations. The non-sexual version is useful after saying something that could be interpreted that way, and the sexual version can make somethign that isn't already that way, that way! Very useful.
Its verb form is very useful in letting people know what you're doing, so they'll leave you alone (or not) while you do it.
The noun form is harder to use, but it's come up before..
"Please tell me that's not your seven poking me in the leg.." "Relax, it's just a flashlight."
See? Useful!
Sigh... I'll just skip the code, I think. "Seven" is too vague. I prefer less clandestine terms!
- Kemrain the Seven.