Looks like a kind of buckthorn to me, but I could be wrong. We have the native, non-invasive Carolina Buckthorn here and I don't think that is it -- leaves aren't exactly like those, though close -- but there is at least one species in the eastern US that forms very invasive clumps. Look up Buckthorn (or Rhamnus species) in a Google image search and see if any of those look like your tree. If it is a buckthorn, the berries are edible (at least those on our variety are) and birds love them, but personally I think they taste really awful. I would eat them in an emergency, but I think I would rather eat grass or bark first!