Did the person from the theater group explain the reasoning? Did s/he say that costumes are rented/ordered from outside, or that there are already people doing the work? Maybe there’s a simple explanation and it’s not personal…or maybe that person is wrong. Can you find someone else from the group to ask? Maybe you would get a different answer from a different person. Or maybe you could volunteer in a different aspect of behind-the-scenes theater and work your way into costuming. If it’s something you really would like to do, I’d explore it further. Maybe stop in to talk to a theater professor?