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.
Jon Broderick
Guitar Tricks Instructor
www.GuitarTricks.com - Home of Online Guitar Lessons