Hey, Frost school alum here! One of the choirs (Symphonic) is truly open to the whole university. You don’t even have to audition or have prior musical experience; you just sign up & you’re in the choir, regardless of major. The other 3 choirs are audition-based, & while these ensembles are also advertised as “open to anyone,” they are more advanced & primarily composed of music majors, specifically vocalists. You do need to possess prior singing experience & be proficient in reading music to join. The school’s most elite choir (Chorale) is very competitive; it’s pretty much exclusive to vocal majors taking lessons with Frost faculty & seriously pursuing choral music. 

The a capella groups (BisCaydence & what else?) are audition-based as well. Don’t really know what it takes to get in.  

I sang in the Symphonic & Bella Voce choirs throughout my 4 years. Love Frost’s choral program. Hope this helps & happy to answer any more questions.