Don, have fun!  Playing in different keys takes practice and it's a great skill to acquire.  

What Dusty said.  Blend in, have fun, join the songs you can play and sit out the songs you can't. 

Since you know your jam partners, it should be easy to ask them what songs they like to play and what keys they play most often :-)  Pick a few songs and practice them before the jam.  If you can only play along on one or two songs the first time you sit in, that's still a good start!

How are the ukuleles tuned?  If they're in GCEA, chances are they're playing most songs in the keys of C and G.  Those are the keys a beginner learns first.  The key of D is only a little harder; beginners won't be happy if you keep calling songs in D but intermediate players should be able to cope.

My solution for playing with ukes is to bring one dulcimer tuned GDG for the key of G and capo at the 3rd fret for the key of C.  If I didn't have a dulcimer that tunes easily to GDG, then I would tune a standard dulcimer down to CGC and capo on the 4th fret for the key of G. 

If the song's not in C or G, I'd sit it out ;-)  Or pick up a uke.  You know those ukes are going to suck you in eventually

The most common keys in folk or old timey or bluegrass music are C, D, G, and A.  You should be prepared to play in those keys in any multi-instrument jam.  And many songs have a specific tuning that they are always played in.  St. Anne's Reel is always in D, Billy in the Lowground is always in C, Red-Haired Boy is always in A, and so forth. I try to practice songs in their common key so that I am always ready to play with others.

How to get into those keys?  I can bring a single dulcimer to a jam.  Tuned DAA or DAd, you can obviously play in D. With a capo at the third fret you're in G and at the fourth fret you're in A. Then you can tune down to CGG or CGc to play in C. 

There are other ways of getting those keys (like tuning DGd for G), and if you have any extra frets playing in other keys gets much easier, but I find the capo is all I need to get the four most common tunings.

But the most important advice I can give is to remember that when you play with others, your job is to blend in, not stand out.  If you don't know the song very well, just play quietly using a minimal number of notes. Little by little you'll learn more of the repertoire.  But you should always listen more than you play. 

And it can't hurt to take the lead on one or two tunes.  Don't be afraid to just say "How about Goin' to Boston in the key of D?" and then start playing?  Others will follow along, and if you show them patience as they learn one of your tunes, they will likely return the favor.

--
Dusty T., Northern California
Site Moderator

As a musician, you have to keep one foot back in the past and one foot forward into the future.
-- Dizzy Gillespie

Seriously?  This is why other musicians groan when you bring a dulcimer to a jam.  We expect everyone to change for us.  How nice? Not.  If you plan to play a dulcimer with other instruments, you should change for them, after all, there are more of them than there are of us.  Majority rules in a jam.

Yes, the dulcimer is a wonderful instrument,..., that can change and play nicely with other instruments.  (And when you get REALLY good, you can bring just one instrument, or a fully chromatic dulcimer,..., hmm, fully chromatic CGg with a capo, that could do most everything.)

Just go ahead and play noter/drone whereever you're tuned - most guitar/uke players will quickly find the key and join in. I was surprised to hear someone playing drone style with a guitar player playing with full chords, but by God it worked and I thought it sounded great. The dulcimer is just the coolest instrument. I someone else if playing the dulcimer, just let them start, find the first chord and your off. It's fun and it's easy, don't complicate it lol.

So——the diatonic nature of the dulcimer tunes it to a specific key. While the chromatic guitar and ukulele can play in different keys without retuning.

Tune as you always do.  Tell them, with their chromatic instruments, that you play in the key of D (or C if you tune CGG or CGc).  They can play to your tuning easier than you can tune to their playing.  

You won't necessarily be able to play every note of tunes they play.  But if you listen, you'll be able to hear 3-5 note phrases of the tune.  If you can quickly find that phrase on your melody string, just play that phrase over and over, in the same time as the tune, and you will have created a "part" for the tune that will meld with what they are doing...

If you say "I'm tuned DAA".  They will tell you that you are playing in the key of d because that's the first note of your scale.  If you are tuned DAd, they will say that you are playing in the key of g.  That's because dulcimer folks tune the instrument to a keynote and everything revolves around that keynote; where chromatic instrument players tune to some base tuning from which they can play in all sorts of keys.

updated by @ken-hulme: 12/18/17 06:53:13AM

Bring two one DAd/DAa and one CGc/CGg and a Capo.  (DAd/DAa depends on your playing style.)

If I get to jam with my son and his guitar and my wife, daughter and granddaughters and their ukuleles....how should I prepare? How should I be tuned?