Tom, Dick, or Harry
A common, undistinguished person; any manner of person, indiscriminately. (Usually in the form "(just) any Tom, Dick, or Harry.") You don't want just any Tom, Dick, or Harry coming to your performance, but then you don't want to limit the amount of business you might bring in, either. Kate's being very selective as to who gets invited to the wedding, as she doesn't want Tom, Dick, or Harry turning up.