In May, Puerto Rico is warm, humid, and marks the beginning of the rainy season, with hot days (highs around 87°F, lows around 76°F) and short, frequent afternoon showers that don't typically ruin days, leaving plenty of sunshine and warm seas (around 80°F) for activities, but it's best to pack light rain gear alongside your sun protection as the humidity is high.
In San Juan or other major cities the ride sharing apps are available, however on the West Side of the Island, where the wedding is going to take place it is very limited and in some places not available. Highly recommend renting a car during your time on the Island.
No, Puerto Rico is a U.S. Territory. No US passport is needed, you only need your Real ID.