One option might be to require the ship carrying the equipment to have a number of ECM equipment boxes equal to the number of ships it's covering (including itself) multiplied by the range to the farthest ship it is covering. So a ship that wanted to jam for itself and two other ships (3 total), one of which was 2 hexes away would need 6 ECM boxes. If it wanted to jam for only the two other ships but not itself, it could do so up to 3 hexes away.
If you want to be more restrictive, you could require the jamming ship to have enough ECM boxes equal to the sum of the ranges to all the ships it is jamming for (counting itself as range 1).
For damage, I'd treat an EW equipment hit like a drones hit (1d6 boxes eliminated for each hit).