They all have quite a bit to do in that show. Most of the girls have around 8 costume changes in the show. Becky Gulsvig's track, for example, has different costumes for each:
Omigod You Guys
What You Want *Different wig*
Positive
"Harvard Party Scene" *Different wig*
Back into the "Positive" costume for "Chip on My Shoulder"/"So Much Better"
Whipped into Shape *Different wig*
Take it Like a Man *Different wig*
All courtroom scenes *Different wig*
Graduation
Finale costume.
That's a heck of a lot of costumes and wig changes. Most of the girls have a similar amount of costume/wig switches. Even Andy Karl has a lot, even though he's only listed as playing Kyle/Dewey/Grandmaster chad, he appears in the background in "Serious", "What You Want" (Not just as Chad, but as a dancer at the end too), most of the Harvard scenes, and "Whipped into Shape".
"Get mad, then get over it." - Colin Powell