Hi,
For this, you'll need to use triggers. either the grenade as a trigger ( a bigger sphere collider) or each GO that coul dbe affected by a grenade would have that bigger collider to know it's in range. I would first go for a grenade based trigger tho.
then catch all trigger enters and trigger exits and maintain a list ( using arrayMaker, you know this tool right? ) to know what object to affect.
Bye,
Jean