QUOTE(briantrumpet @ Aug 7 2008, 10:50 AM)

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The other thing I'd add, at the risk of stating the obvious, (and you probably are doing it anyway), is to listen to as many recordings of pianists in jazz enembles as you can. There's heaps of stuff on youtube - and you can start to work out the differences between solo jazz piano styles and what's necessary in a jazz group. For instance, if you've got a bass player, you won't have to worry about doing a constant bass line, etc.
Brian's right. To add to that I'd say you're going to have to figure out two thing - how to 'comp', which is basically putting the chords in with correct voicings and rhythmic feel while other players are taking it in turns to play solos, and then learning to improvise yourself for when it's your turn to take one.
In other words, if you don't know how to do either before you join the ensemble, and the other people in the ensemble can already improvise and will be needing you to 'comp' at the very least, then you will struggle. Definitely best to come prepared, especially with the comping side of things.
Once you've got it figured you will have fun though!