[Threshold]

Set them to the same team.

https://sampwiki.blast.hk/wiki/SetPlayerTeam

Quote:

Players can not damage/kill players on the same team unless they use a knife to slit their throat. As of SA-MP 0.3x, players are also unable to damage vehicles driven by a player from the same team. This can be enabled with EnableVehicleFriendlyFire.