How do I make this script only run on player join?

Connect a function to OnPlayerAdded. This makes it run when a player joins.

player = script.Parent.Parent
char = player.Character
cam = game.Workspace.CurrentCamera  
char.Humanoid.WalkSpeed = 0
game.Workspace.CurrentCamera.CameraType = "Scriptable"
wait()

local Players = game:GetService("Players")

game.Workspace.CurrentCamera.CameraType = "Track"

intro = game.Workspace.Intro
pos1 = intro.IntroCam1
pos2 = intro.pos2
pos3 = intro.Pos3
pos4 = intro.Pos4
pos5 = intro.Pos5
pos6 = intro.Pos6
pos7 = intro.Pos7
endpos = intro.EndPos

function onPlayerAdded()
    game.Workspace.CurrentCamera.CameraType = "Scriptable"
    char.Humanoid.WalkSpeed = 0
    game.Workspace.CurrentCamera:Interpolate(CFrame.new(pos1.Position),CFrame.new(pos1.Rotation),2)
    wait(2)
    game.Workspace.CurrentCamera:Interpolate(CFrame.new(pos2.Position),CFrame.new(pos2.Rotation),3)
    wait(3)
    game.Workspace.CurrentCamera:Interpolate(CFrame.new(pos3.Position),CFrame.new(pos3.Rotation),1)
    wait(1)
    game.Workspace.CurrentCamera:Interpolate(CFrame.new(pos4.Position),CFrame.new(pos4.Rotation),5)
    wait(5)
    game.Workspace.CurrentCamera:Interpolate(CFrame.new(pos5.Position),CFrame.new(pos5.Rotation),3)
    wait(3)
    game.Workspace.CurrentCamera:Interpolate(CFrame.new(pos6.Position),CFrame.new(pos6.Rotation*90),5)
    wait(5)
    game.Workspace.CurrentCamera:Interpolate(CFrame.new(pos7.Position),CFrame.new(pos7.Rotation),3.5)
    wait(3.5)
    game.Workspace.CurrentCamera:Interpolate(CFrame.new(pos7.Position),CFrame.new(pos7.Rotation),1.5)
    wait()
    char.Humanoid.WalkSpeed = 16
    game.Workspace.CurrentCamera.CameraType = "Track"
end

game.Players.PlayerAdded:connect(OnPlayerAdded)

You have it running for each and every player at once. Instead, connect the function!

At the bottom, put this:

game.Players.PlayerAdded:connect(onPlayerAdded)