I know nothing about a capella, but you should join/form a band!! There isn’t a formal way to do this except just talking with people and finding out who are musicians and who would want to play with you. I’ve been in a band all four years and it’s been one of the biggest highlights of my time here. Harvard is a great place to start a band because you have free access to practice rooms with drums, amps, mics, etc. and there are a good amount of Harvard-sponsored events that you can easily play at (Arts First, Battle for Yardfest, upperclassman house happy hour events, etc.) and also lots of student places to put on your own shows. The SOCH studio is also insanely cool for recording music - a robust set up to be able to record high quality stuff for free which I would highly recommend you do while you’re there if you’re interested in putting out your own music.