How do I play with guitar players at a folk song/60's night?

Hi All,

I'll bet this has been addressed here before but I haven't found the thread just yet.

I am a fairly new player. I have been invited to bring my dulcimer to an evening where we will be singing songs from the 60's. I've seen the fake book (new term for me to use!) and it will show something like "Jambalaya" and above the words there will be a letter like "D" which I am assuming is a D chord.

What do I need to know about strumming/tuning/playing with the singers and guitar players in the group?

There's no key signature shown on the lyric pages. How do I know what key everyone is in? Any hints on when I should sing and NOT play?