We're still not quite sure what sort of tunes or music we are talking about which makes it a bit tricky.

I agree that if someone wants you to play with them it's courteous to offer a guide to chords unless you play with a group of people who just expect you to know. If you are playing in a less formal environment it's sometimes fun to experiment and play chords that aren't necessarily 'right' but create a tension in the music that either works or doesn't. People react one of several ways to that - they either tell you you are playing it wrong or go with the flow and experiment.