It would depend on how your grenade works. Does it explode on impact or is it timed? How are you determining the radius of the grenade?
This is just a sample - you can use a collision sphere to set the radius.
Then however you have your grenade exploding, you would run a server event to get overlapping actors of whatever your player pawn’s class is (mine is PP_Game in this instance) and iterate through and call the Apply Damage node.
Then on your pawn you would use the Event AnyDamage node to do whatever you want with the damage (i.e. subtract it from health, show screen effect on client side, camera shake, etc.)
Hope this helps 