When I play with guitar players, I usually try a couple of things: Key of D (in either DAd or DAA tuning) and Key of G using DGd tuning.  D and G are common and comfortable keys to play on guitar.  If your guitarist wants to play in C it's easy to capo the guitar up one step (which puts it in D) and you play the dulcimer in D (again using either DAd or DAA tuning).

If I am doing a mix of chord style and noter-drone in the same session, I like to use DAd for chord style and DGd for noter drone because you can quickly re-tune that middle string from A down to G and it's easy to hear the perfect 4th between the D and G strings.  It sounds like the first two notes of Here Comes the Bride.

Hmm...wondering if you could tune your bass and melody strings down a bit to C (middle string up to C), and then use a pencil or dowel to make barre chords across all your strings, and just play the melody if the songs are following a familiar structure, or else find your three I IV V main barre chords that match the tune's key.  Might not take long to figure out the key, and by knowing your zero fret is C, you can find the tonic barr chord right away and you're in business.  This could work fine especially since you have the 1.5 and 6.5 frets.   It'd be less to think about, no chord fingerings to keep remembering and switching, and might work well for that unusual situation, surrounded by key-changing guitars all night. 

Another option is to tune to CGc and do the same, making barre chords after you locate the tonic location for each tune they play.   Doing from the home base of C would be easier to calculate on the fly than starting in D on the open strings.

You can play the I IV and V chords for the keys of D, A, and G in DAA tuning. The "extra fret" is helpful, enabling you to play an E chord in the midrange of the fretboard.  So if you are tuned DAA,  The I IV and V chords are D, G, and A, respectively for the key of D.  If the other jammers switch to G, the I IV and V chords are G, C, and D, respectively.  Make the C chord on frets 6,4, and 6.  When they switch to A, the I IV and V chords are A, D, and E, respectively.  Make the E chord on frets 5,4, and 6+, from bass to treble (far to near).   Fiddle and banjo players love the "sharp" keys, so this knowledge will carry you far.  If you find your jamming mates want more keys than that, an extra instrument is advisable.

The capo allows the guitar player to play in D with the same fingering that would be used if the music were played in C.  Another option would be for you to drop the pitch of your dulcimer down a whole note to match everyone else playing in C.  Have fun!

As long as they are playing in D. And you are tuned DAD.You can follow along by only freting a single note for each chord. So when they play the A chord you only fret the A note and strum the remaining strings open. You can't go wrong that way... Later on get the guitar player to retune to open D.. or DADF#AD and let him play along with you doing the lead...Bob.

Thanks for the advice! I managed to play along a little on a couple of songs, I let them know I was in the key of D, and they let me know what chords they were using. I also managed to play about four songs, though I messed up on each and every one, I got through them OK. I swear I play them all flawlessly for the cats about a dozen times a week! Funny thing is, when I picked up my dulcimer to practice later, I was all thumbs.

It was a really fun time, so I'll just keep practicing and look for any opportunity to play in front of people.

This helps me (but it may not help you). My brain sort of thinks backwards sometimes. Anyway I recognize a bunch of guitar chords by sight just from watching guitar players play (and I play a bit too). So if I have my banjo or my dulcimer I'm watching the guitars in the room for the chord changes.

Good luck!

Beth,

Knowing I, IV, V chords in the keys of D, G, C will take you far. Listen for the chord changes and key off whoever looks to be the most solid rhythm player.

Have fun!

PS-My guitar-playing husband helped me with this. :)

A friend of mine is hosting a party room at our local Science Fiction & Fantasy Con which includes a music circle. Everyone else will be playing the guitar, and I will be playingmy mountaindulcimer. I do have extra frets, 1 1/2 and 6 1/2, and I know basic chords. And I do have a capo.

Any suggestions on how I can play along?