What sort of grenades are you thinking? My vision of mobile drone defense is two-tiered: the outer perimeter system finds drones large enough to have a radar signature and attacks them with proximity fuzed grenades. I read about this
Northrop Grumman solution a while ago.
The inner perimeter is trickier - it would have to cover small drones not picked up by radar. And without a radar signature, proximity fuzed rounds do not work. I suppose an optical rangefinding system could be coupled with time fuzed rounds? Other than that, a shotgun-like multiple pellet projectile is likely the best option.