Playing dulcimer with a ukelele
Playing and jamming difficulties...HELP ME!
Contact the club and ask if they welcome other instruments. A few clubs are doctrinaire about ukes-only; you wouldn't want to join them anyway, they're no fun! Then ask for a copy of the songbook. Like Dusty says, you'll want to study up in advance. Look at the chords you'll need... and DO NOT PANIC. You don't have to play on every song. And you don't have to play every chord; you can mute the strings when there's a chord you don't know.
A beginner-friendly club in the US (where ukes are generally tuned GCEA) will mostly play in the keys of C and G. If you show up ready to play these six chords you should be in good shape:
It's nice to add the 7th chords (C7, G7, Em7 and so on) but really they are optional. The uke players will be adding a 7th note to some of their chords and it won't matter if you do; the notes you're playing are compatible. You could learn the theory behind this or just take my word for it.
I play all of these chords on an MD tuned DAd with a 1-1/2 fret and 6-1/2 fret. If you don't have a 1-1/2 fret, you can tune to CGC as Ken suggests. Another option is to tune a short-scale dulcimer(or a baritone) to GDG which puts you in the key of G; capo at the 3rd fret for the key of C.
We're all assuming you play chords. When in Rome... strum chords and sing like the uke players. It would be difficult (not impossible) to fit noter/drone into a uke club. If you ever try it and the drones aren't working -- which they probably aren't -- you can stop playing the drone strings and play single-note melody or harmony.
If this all seems too overwhelming at first, go visit a uke club just to listen and sing along. Talk to people and you may find others who play dulcimer (or would like to learn). Even if you decide the club isn't a good fit for you, it can still be a good place to meet other musicians.
CAUTION: No matter how much you protest, they will force a uke into your hands and show you how to play a C-major chord. My local club has two loaner ukes and we force them on restaurant customers, passersby, wait staff and children. I swear there are neighborhood dogs who know how to play a C-major. Ukulele Acquisition Syndrome (UAS) is a dangerous malady made even worse in conjunction with DAD/DAA. Do not ask me how I know this. Save yourself.