After concerts many performers make themselves available to meet audience members, sign autographs, and chat. Some members of orchestras and chamber groups really like to hear from the audience. Even some of the most famous conductors, singers, and instrumentalists may take time to meet listeners. Whether or not the musicians are famous, this can be your chance to tell a performer how the music affected you, or even to meet a composer.
Of course not every musician is willing to meet the public, and sometimes another engagement prevents a performer from staying around after the concert. But it's worth a try. Leonard Bernstein, for instance, would spend as much two hours greeting listeners after a concert.
Ask an usher where to go to meet the musicians, or look for the stage door or artists' entrance. You may have to wait for a while, but the fun of meeting the performers can be worth it.