If your invitation included an option for a Guest then yes, if not, we would prefer if it was just you.
Formal to semi-formal. Suit jackets, vests, and ties are welcome but not required. Consider knee or floor length dresses, skirts, blouses, etc. In early May it will probably be warm, but could get cooler as the night progresses, so layers might be a smart option. And don't forget your dancing shoes!