SATURDAY MORNING UPDATE: This morning I e-mailed the Clinton campaign to find out more about this pledge. I wrote: "Am interested in learning more about the senator's plan to give residents of Guam the right to vote in presidential elections. Would that mean making Guam a state? Bestowing citizenship upon these US nationals? Same with Puerto Rico, the Virgin Islands and American Samoa? Would they get Senators and full voting members of Congress? How about DC? Wouldn't this require major constitutional changes?"
People born in PR, the US Virgin Islands, and, yes, Washington DC are born US citizens, though people from American Samoa are not.
...adding, since it wasn't crystal clear in the post: People from Guam are born citizens too.