What is the best way to protect from groin shots in paintball ?

Wear a cup.

In all honesty, it is an unlikely scenario unless someone deliberately tries to hit you there. Most of the time it is head or upper body shots because that is what is typically exposed. Perhaps one of the best things you can learn is being aware of what you are exposing. All it takes is leaving an elbow or maybe the top of your head sticking out and you are walking.

I would just wear a cup.

Just suck it up. I've seen guys wear their pod packs in front of their groin, but I think that's ridiculous. There's very little chance of getting hit there, and if you do, just suck it up and keep playing.

Shoot first and fast.

You could wear a cup I suppose.  Though it's pretty unlikely you'll ever be shot in the groin, and if you are, it's really unlikely it'll happen more than once.

Don't lead with your groin.

Either a cup for your balls or castration. FACT