They give themselves a lot of freedom, the only suggestion required is the name of the musical they create. Any of you who have seen the Impro Musical, Impropera or the Comedy Store Players doing their "Musical Adaptation" game will know the routine.
My main criticism would be that there were sometimes too many of them on stage at once - things got a bit cluttered at times and you were finding yourself having to concentrate hard on what was going on. Also, as is often the case with impro singing, there was a lot of out-of-tune singing, especially in group songs, where shouting out of tune seems to be the accepted alternative to finding a harmony. Perhaps some singing lessons would help. Despite this, some of the solos were as good as you'd expect from a pre-written musical - lots of different styles of music - one piece that sticks out in my mind is a country and western ballad in a Tammy Wynette send-up style.
Special mention goes to Larrance Fingerhut - Musical Director and Accompanist. The people at the piano never get sufficient mention it seems, and this one was one of those impro musicians who are so good at their job you forget they are making it up.
I'd recommend this for all of you - from the die-hard impro fans who like to see a good, well crafted piece of long-form impro to the "I used to watch that Whose Line thing" crowd who want to see something spontaneous and new.