Dennis,

It's hard to give you some specific songs because "bluegrass" consists of so many different genres of acoustic stringed music. The songs you hear at a jam session vary so widely based on region, age of the musicians, and especially the skill level of the folks at the jam session.

It can even be affected by a certain "herbal" aroma that sometimes permeates the session.

The advice that Andrew gave was very good and would serve you well.

If you'd like some very good book CD/DVD lessons on bluegras jamming, Homespun has an excellent selection by some masters of bluegrass music. Some of the lessons that focus on jamming can be found here:

http://www.homespuntapes.com/shop/category.aspx?id=330

Others can be found by clicking on some of the links. Some focus on instrumentals while others focus on old standards with lyrics.

Good luck and have fun.

Bob
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http://soundcloud.com/bobcflatpicke/music-in-the-mountains