I'm finding myself in a bit of a rough spot here.
 I have some friends who play music, fairly distortion-guitar-heavy stuff--think Queens of the Stone Age meets the bassist of Death From Above 1979.
 Anyway, they know I play piano, and asked me if I'd like to jam with them. They said they'd love to have some "keys."
 Now, all I've ever played on piano is classical, a few video game songs, and a bit of jazz. Also, I'd never played piano/keys with people, only drum set and bass guitar.
 Now my problem is this: When I went to a jam session of theirs, I was lost. Clueless. I knew what they were playing; I knew the chord progression. I just, for the life of me, could not figure out what I could do which would fit.
 I've got my Casio PX-3 and a synth at my disposal, so the voices available are dauntingly flexible as well. However, that's a thought for after I figure this out.

I understand that this is not the kind of issue that anybody could really answer and ultimately is down to practice, but any sort of insight--ideas, suggestions, artist/song example suggestions to listen or learn, comments, questions, jokes--I would greatly appreciate. I just want to find some sort of grounding in the style so that I can get on a track to get a bit of myself out in the music.

I am shy.

Thank you for reading.

Most popular music is rhythm driven. In that style you basically pound those chords into submission! If you can make distorted organ sound play some pentatonic riffs over the chord progression. Try listening to the music a lot and shortly you will soon get a feel for it. Grunge and Alternative are simple songs played with a lot of emotion.

I had the same experience many years ago when I was tagged the keyboard guy in a band simply because I played piano, mostly classical. The chord progressions were simple, everything should have been simple yet I was lost. Being naturally shy sure didn't help either. What I eventually learned: 1) There are a fairly limited number of "standard" rock/blues moves. Listen to the music, you will hear them over and over again. They are easy to imitate. 2) The major and minor pentatonic scales will carry you through almost any "soloing" you're called upon to do. 3) give up precision, some slop is called for 4) not everything you do has to make sense, at least immediately 5) if you've got organ tones sometimes you don't have to do much of anything except hold that chord then slide into the next one 6) it's only rock and roll, so stop worrying about doing it "right" and just do something that follows the rhythm. The shy thing is a problem in rock, you have to allow or make yourself become someone else in that arena. It's ok, it's expected, everyone does it.

If the others of your band agree on the style, I would in general first try it with a slightly distorted B3 organ sound with quite some reverb mixed to it, keeping a chord, vary it by chord inversions, change from time to time the Leslie effect, even try to use a Wha or flanger effect from time to time (intensity of this latter effects could nicely be controlleld by the dynamics of your touch, or the aftertouch function of your synth keyboard).
Maybe listen to the intro of this, to get an inspiration:
http://www.youtube.com/watch?v=gZ_kez7WVUU
Maybe listen to the last third of this one as well, to get an inspiration:
http://www.youtube.com/watch?v=9_Iq9CWuqMM&feature=related
More modern tunes you might want to consider for inspiration:
http://www.youtube.com/watch?v=5fhXGSUQO3w

To me the basic of the keys in this genre is, that the organ adds sound to the music, so that the concert hall becomes really saturated, but does not add the melody. The latter is part of the voice and the lead guitarre.
The piano will wake up the audience hidden desire for classical music, but be careful to know your audience well to not overrun them - give them the feeling, but don´t require them to quietly having to listen and having to search for sophisticated pralines: rock audience wants to jump and scream. Help them do so!