What sites have the "Challenge a friend" option?

[BigBrother]

Related question about Party:

Can your friend sit with you at a private table if they are NOT allowed to sit with you at a public table?

I assume that if Party is blocking us from sitting together then they would do it across the board, but it would be nice to be able to share a friendly game occasionally.

[Seether]

You can sit with anyone on private tables on party (including those you cant normally sit with)