This list isn't real short but I've listed out instrumentals and songs that I've found most people know. They aren't all necessarily bluegrass but I've found that bluegrassers have often heard these tunes and I've also discovered that jams may be made up of old-time pickers or folk singers or whatever.
I'd recommend learning the chords to all these tunes. You don't have to play a lead in a jam session but you ought to do backup well enough to make the other person sound good. WARNING: If you do want to play a lead, you probably ought to start the song that way it won't go way too fast. You'll still start it quicker that you wished you would have, but not nearly as quick as someone else ;-)
The instrumentals are usually played in specific keys, especially if there are fiddlers or mandolins. Songs are sung in whatever key the singer chooses. If you really like one, then you ought to sing it and make everyone follow you. That may seem disconcerting at first, but it's actually easier if everyone at the jam is better than you. Then you're not trying to keep up with them, they're just picking behind something you already know.
* The tunes marked with asterisks have more chords than seem reasonable so they're a bit trickier than the others.
Big Sandy (A)
Bile Dem Cabbage Down (A usu)
Bill Cheatem (A)
Billy In The Low Ground (C)
* Blackberry Blossom (G)
Cherokee Shuffle (A)
Cripple Creek (A usu)
* Dear Old Dixie (G)
Devil's Dream (A)
Down Yonder (G often)
Foggy Mtn Breakdown (G)
Lonesome Fiddle Blues (Dm)
Old Joe Clark (A usu)
Rag-Time Annie (D)
Red-Haired Boy (A)
Redwing (G or C often)
Saint Anne's Reel (D)
Salt Creek (A)
Soldier's Joy (D)
Turkey in the Straw (G or D often)
* Whiskey Before Breakfast (D)
* The tunes marked with asterisks have more chords than seem reasonable so they're a bit trickier than the others.
Amazing Grace
Dark Hollow
Goin' Down the Road Feelin' Bad
Hand Me Down My Walking Cane
Long Journey Home
I Am a Pilgrim
I Know You Rider
I Saw the Light
In the Pines
Mountain Dew
Nine Pound Hammer
Old Home Place
* Pallet on Your Floor
* Rocky Top
Roll in My Sweet Baby's Arms
Salty Dog
Sitting On Top of the World
Swing Low, Sweet Chariot
Think of What You've Done
Tom Dooley
Way Downtown
Will the Circle Be Unbroken
You Ain't Going Nowhere
* The tunes marked with asterisks have more chords than seem reasonable so they're a bit trickier than the others.