The G1 is currently the only grenade in the game. All CATs are equipped with a grenade launcher. If more than one enemy is within the splash radius of a grenade when it explodes, they will both be targeted.
By default, a grenade can be launched with the middle mouse button. A grenade can be charged to make the grenade fly farther. After you have launched a grenade, there will be a 5 second delay before you can launch another.