Basically I would try to not play out of key. First, get them to show you a song they do. Learn the basic chords. Then, try to find a simple riff or synchopated chord progression that you can play in each song.

Lots of times I just play one note funky style until I get the hang of it. Or just arpeggiate the chords they showed me.

I wouldn't try to show technical skill except in a lead. Instead I would try to show musical catchiness.

Also, if everyone is playing complext stuff, play simple. If everyone is playing simple, play complex.

Hope this helps.