Playing dulcimer with a ukelele
Playing and jamming difficulties...HELP ME!
@katiemac225, Ken has explained the central issue here. The ukulele is a chromatic instrument, so it can play in any of the 12 keys. The dulcimer is a diatonic instrument, so if you are tuned DAA or DAd, it will be very easy to play in D (or Bm), somewhat less easy to play in G or A, and very difficult or even impossible to play in other keys.
What does this mean in practice? I would suggest two approaches for you.
First, you might find out ahead of time what tunes the ukulele group plays. Most groups use a songbook. Get a copy of that book and look through it. Find the tunes in the key of D and expect to play along with those, skipping the rest (for now). [You can play in C if you retune to CGc or CGG, and out of your D tuning you can use a capo at 3 to play in G or at 4 to play in A. So you can start to add the tunes in those keys as you get comfortable.]
Second, you might approach one or two of the friendlier, patient people from the ukulele group and ask if they would play with you. It will be easier to ask one or two people to play only in D than it would be to get the whole group to change their routine. As you get comfortable playing with those one or two people, you might then be able to join the group.
In anticipation of playing in either of those scenarios, you can practice by getting used to strumming chords and singing songs, for that's what people do in uke groups. They either use a songbook or lyrics with chords are projected on a big screen (sometimes with a strumming pattern indicated as well) and they all strum chords and sing together. Try that for yourself. You might start with two-chord songs like "Jambalaya" and then move on to three-chords songs like "Jamaica Farewell" and then four-chord songs like "Let it Be" and so forth.