An enlightening thread has popped up on the with the header,

Just so everyone's on the page: Every Tuesday at the UCB, , a weekly dedicated showcase of the art of improv. A casual theatregoer at a Harold Night might assume that these teams form naturally--8 friends get together, start a group, ask for some stagetime, and there you have it. But, in actuality, the team making process for many of the theatre's groups is constructed by a creative panel.

explains it all:

Harold teams formed under the auspices of the UCB don't have full control over their membership and fates. They are initally formed by a committee at the UCBT, usually through an audition process. If the creative leadership at the UCBT feels that a team's roster needs to be revised or disbanded or that the entire roster of harold teams need to be shaken up they do that. The teams, therefore, are pretty transitory structures. Sketch and improv groups that are formed by their constituent members tend to last longer.