Keeping teams even even if player joins late?

function getTeams()
	local teamNumber = 1
	repeat wait()		
		local lobby = {}
		local teams = {"Red", "Blue"}
		for _, player in pairs(game.Players:GetPlayers()) do
			if player.TeamColor == game.Teams["Lobby"].TeamColor then
				local character = player.Character
				if character and character:FindFirstChild("Torso") then
					table.insert(lobby, player)
				end
			end
		end
		
		local randomPlayer = lobby[math.random(1, #lobby)]-- Error here (Line 77)
		randomPlayer.TeamColor = game.Teams[teams[teamNumber]].TeamColor
		toMap(randomPlayer)
		
		local lobbyNumber = 0
		for _, player in pairs(game.Players:GetChildren()) do
			if player.TeamColor == game.Teams["Lobby"].TeamColor then
				if player.Character:FindFirstChild("Torso") then
					lobbyNumber = lobbyNumber + 1
				end
			end
		end
		teamNumber = teamNumber == 1 and 2 or 1
	until lobbyNumber == 0
end

ServerScriptService.GameScript:77: bad argument #2 to 'random' (interval is empty) The code here is suppose to get all the players in a game and give them all random teams and keeping it even. It also is suppose to give a team to any new player who may join late, but still keep the teams even. For some reason it is error at local randomPlayer = lobby[math.random(1, #lobby)] and idk why. It seems to work if you have say 4 players in the game and the starts, but if another play joins mid game the output has that error. How can I get it to just add the player to the game?

Since a new server starts before a client(the player) joins, we need to wait until a new player join. Also, I think some parts of your script are unnecessary.

local CurrentTeam = 'Red'
local lobby = {}

game.Players.PlayerAdded:Connect(function(plr)
	table.insert(lobby, plr)
	--I'm not really good with teams, so just do whatever you do to turn this player's team color to the color of CurrentTeam
	if CurrentTeam == 'Red' then
		CurrentTeam = 'Blue'
	else
		CurrentTeam = 'Red'
	end
end)

game.Players.PlayerRemoving:Connect(function(plr) --You know, what if a player decides to leave?
	for i, v in pairs(lobby) do
		if v == plr then
			table.remove(lobby, i)
			if CurrentTeam == 'Red' then --so the even team thing does not mess up
				CurrentTeam = 'Blue'
			else
				CurrentTeam = 'Red'
			end
			return
		end
	end

	warn('Oh no! We could not remove a player from the lobby table!') --Runs if it never finds the player.
end)