How do you make a weapon that only hurts one team?

It's a single line, you just need 2 Players, both of them have Team property which says in which team they are, you need to compare player that shot the bullet or threw the rock's team with player who got hit's team, if they are in the same team, don't dare you damage him.

local player_that_attack = [...]
local player_that_got_hit = [...]

if player_that_attack.Team == player_that_got_hit.Team then
    print("u try to attack team member")
else
    print("kill this pig")
end

In order to check for a different team you can use the same method:

local Teams = game:GetService("Teams")

local target_team = Teams.[...]

if some_player.Team == target_team then
    print("Deal damage")
end