While it @MonkeyX makes sense, the effect will be applied to the whole Blast range, and then it raises the question - can the grenade land in each's guy pocket, in the blast range? Of course, we can rule that it lands at an exceptionally good angle which maximizes the fragmentation (and close to the desired spot)...
I think I might go with @RogerMc's interpretation - only direct hits with weapons that detonate/activate on impact have effect bonus. The rest targets in the blast range will simply take the damage. Of course, with grenades - the higher roll, the closer to the desired spot.