I am married in Guatemala; now I want to marry someone in the US. Is this legal? Will this marriage be valid?
No. In order to enter into a valid marriage, you must first dissolve your existing marriage.
Child Custody Lawyer
No, you cannot get married in the US if you are already married, no matter where the first marriage took place.
Family Law Attorney
If your question is whether, if you've married one person in Guatemala, and she's still alive, and you haven't divorced her, you can still marry somebody else in the United States, the answer is NO.
By the way, if you attempt this, and your new "spouse" in the U.S. doesn't know these facts, you may have some or all of the responsibilities of being married to that second person; i.e., she may be entitled to support, property, etc.
You will also be committing criminal bigamy.