I'm in KKPsi, but thought I'd put my two cents in as well. My understanding is that any person who is an active and enthusiastic band member is technically eligible for membership, and that requirements such as being an actual musician are determined by the chapter. Our current class includes a colorguard member, although she is also a very fine oboe player. However, we invited the entire colorguard to consider joining, regardless of instrumental ability. My belief is that it is the spirit of band involvement and not the type of involvement that matters. Horn players and drummers provide the audio aspects of a band, and colorguards provide the visual. It's a complete package.
Iota Kappa, KKPsi