Jim hit on a key word when he said "generally" quite welcome. The majority of Bluegrass bands/players I've been around are fairly open and accepting, but be aware that not all Bluegrass has an open attitude toward instrumentation. I've been around hard core Bluegrassers where even a well-played whistle would be about as welcome as a tambourine at an ITM session.
BTW, one of the bands I sit in with on occasion is exactly what Thomaston described...Irish/Bluegrass/Old Time with a bit of Zydeco mixed in. Really great fun to play with these guys and they just let me do whatever comes to mind on both guitar and whistle.
![big grin :D](./images/smilies/icon_biggrin_144.gif)