Even though I'm not a big bluegrass fan/player (I started out playing it but quickly developed more of a fondness for Irish, old-time and swing) I must admit that 'grass lends itself to mixed sessions far more easily than most folk genres.

1) A bluegrass "song" can typically be enjoyed as an instrumental piece by the non-singers in the session. Most songs are sung so as to give the pickers a chance to take a break between verses.

2) Conventional bluegrass vocal delivery - the "high lonesome sound" - can carry over a number of instruments. In other genres, it's often hard to hear someone singing over their own guitar.

3) Bluegrass songs tend to be relatively easy to follow. That gives everybody in the session something to do. One of the biggest problems I've seen in song-oriented sessions is singers choosing pieces that nobody else knows and are hard to play intuitively.

4) A large number of bluegrass songs are "standards" that almost everybody knows, so lots of folks get to join in.

5) A number of bluegrass standards have been "covered" by groups like The Grateful Dead and String Cheese Incident so they provide a pleasant common ground between old farts like me and young kids who spend their summers following jam-bands around. (:-)

Bruce