by Andy Sloey:

So you want to start your very own improv team - the first question is usually "Who's gonna be on it?" The answer is deceptively simple - the best people to have on your team are those who you find interesting. People who make you laugh.  People who you genuinely enjoy being around.  Your friends! 

Improv is built on play, and just like in childhood, we're best suited playing with our friends. Don't have any friends in your local improv community?  Make some!  Take a class, go to shows, seek people out. Take note of those who light the fire of creativity in you and mark them as future teammates. Once you've assembled your group, things tend to get a little trickier. You'll need to schedule regular practices and this can prove to be daunting considering your busy high school, college, or work schedules. You may lose people to scheduling issues, and should be prepared to potentially go through several iterations before you find a lasting formula. Don't get discouraged! Finding the formula is a valuable process during which you'll learn a lot about yourself, your play style, and what qualities you value in other improvisers.
Once you've got your perfect team, its now on to building that ensemble! Make practice a priority, get a regular coach to give you an outside perspective and get up on stage. Also, and just as important as practice, is hanging out! Go to the park, dinner, have a party, whatever.  Supporting one another on stage takes a lot of trust and group mind, which is formed organically over time.