I think Elliot and Nanase did it before the comic started. At least that is what "friends giving each-other privileges" implies to me.

Elliot and Sara have probably done it already. But there are no strong indicators one way or the other.

Not like with Tedd and Grace where Dan has all but said they haven't in the Keenspot forum.

I'm going to guess that Ellen and Nanase will be next.

My money is on Justin and Susan. During the party, and possibly on the dinner table...  

Yes I did just say that, and here's why. Justin is about to be female. He hasn't found a man (http://www.elgoonishshive.com/d/20020603.html) , and Susan  will fill that function during the party. Their interests are similar, and it seems like their personalities are a good fit (I'm not going to throw all the Night Out references at you people).

Her side is a bit more complex though. She will be driven by a need (conscious or subconscious) to see her belief about the male of the species be proven true (http://www.elgoonishshive.com/d/20050119.html), and by what will be an even more deeply hidden desire to accomidate her mother's wishes in regard to her (Susan's) sexual preferences (http://www.elgoonishshive.com/d/20030707.html).

Add to those factors the fact that desire follows form as established with the Uryuoms in Tam eh Tedd (http://www.elgoonishshive.com/d/20031006.html), and you have the makings of an interesting night.

and the dinner table? just a guess

  ...Aaaand it's thoughts like that that make me grateful the comic's PG13...thus sparing us from such eye-scarring incidents...