Arty/Mortar FO places a pending fire--that becomes incoming which hits all units on the card (and blocks LOS out of the card, incidentally). All units on the card check for hits with the modifier of whatever arty you called for and any modifiers.
Rifle grenades are just a grenade attack, like bazookas or hand thrown grenades, so are against a single piece of cover or a single unit in the open.
Steve's response is correct. For your future reference, this is explained in section 6.3.2 for all the special types of fire: Grenades, Grenade misses, Concentrated fire, Snipers, Crossfire, Incoming!, etc.