Under assaults against vehicles monstrous creatures it says " a model can use such a grenade as a melee weapon, but can only ever make one attack, regardless of the number of Attacks on it's profile" Which means all models in the unit with grenades that can be used in assaults against vehicles and monstrous creatures can all swing with them in close combat, but each model only makes one attack with them even if they have like 2 or more attacks.
So all models in a unit of space marines with crack grenades in close combat with a vehicle or monstrous creature could all choose to attack with their crack grenades instead of their power axes or whatever. But if they did that each model would only make one attack even if they would normally be able to make two or more with their other melee weapons.
Same thing with melta bombs, every model with a melta bomb can swing with it in close combat but each model would only be able to make one attack using the melta bomb. (Which makes the new tankbustas particularly nasty since each model comes with them by default)
Throwing grenades refers to making a shooting attack with a grenade instead of another weapon. It explains it under each individual grenade's profile. "When a unit armed with krak grenades makes a shooting attack, one model can choose to throw a grenade, rather than using another weapon". The reason it specifies one grenade can be thrown per phase is so if you wanted to say overwatch with krak grenades only one model would be able to snap fire a krak grenade instead of using their other weapon.
