Had quite a bit of experience in university. Before that I was mostly a solo performer, and quite enjoyed it. Playing clarinet in band since elementery school all the way through university Wind Symphony, tremendously helped my ensemble skills.
One of my best friends at University is a fluetist, and we have played much music together just for fun. He was also enrolled in a chamber music course with another flutist, they were just missing piano and he asked me to join. It was a bit confusing for met at first, I got lost in the music many times. As time passed, we advanced as a trio quite successfully. It gave me substantial experience with small ensemble playing (quite different from large orchestral groups).
I enrolled in pedagogy course for my last year at univeristy. I've already taken several of these before. Turns out I was the only one that enrolled in 4th year. Teacher is a well known accompanist in our city. She offerst to teach me accompanying instead. I don't know what to think so I say OK. Now I get myself into this whole different animal of vocal accompanying. Very complex, more complex than accompanying any other instrument. I learn a lot. I'd recommed studying vocal accompanying to any pianist out there. It will change the way you think of the piano, and it's possibilities.
University also was putting on a production of Orff's "Carmina Burana". They were doing the 2 piano, 5 percussion reduced score. They needed a pianist, and asked me. Now this was FUN

. Much learned here with this experience. You have to really follow the conductor. You can't go rambling off like you are playing a concerto or something. The added problem of 2 pianos is the issue of note attack. With a whole buch of strings and winds, if one is slighly off the attack you probably won't notice much. On the piano because of it's distinct attack, you have to play exactly with the other pianist.