Play with ukulele and guitar
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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.

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