Leave When Someone Joins

local Players = game:GetService("Players")
local localPlayer = Players.LocalPlayer

-- Toggles
local kickIfAlreadyInServer = true   -- kicks if someone is already in server
local kickOnPlayerJoin = true        -- kicks if someone joins later

if kickIfAlreadyInServer then
	for _, player in ipairs(Players:GetPlayers()) do
		if player ~= localPlayer then
			localPlayer:Kick("Another player is already in the server.")
			return
		end
	end
end

if kickOnPlayerJoin then
	Players.PlayerAdded:Connect(function(player)
		if player ~= localPlayer then
			localPlayer:Kick("Another player joined the server.")
		end
	end)
end