The Atarinstrument is currently fairly basic, but supports 2 instrumentalists (2 joysticks). Have fun!
I'm thinking I'll probably change the button to be a sharp instead of a flat in the next version and add some more instruments. One instrument I really want to add is the drum kit (down=bass, up=snare, button=hi-hat). The biggest question is how to make the choosing of the instrument intuitive, whether layered instruments should be allowed (via setting sound and doing drawscreen more than once per main loop). I'm thinking of just showing the letters "ATAR" on the screen while you play that are maybe different colors based on the instrument you are playing. Let me know what you think.