How to make teams in Roblox Studio

-- Script in ServerScriptService named TeamManager.
-- Lets players pick a team from the Teams service, keeps teams even, and respawns
-- the player at their team's SpawnLocation.
local Players = game:GetService("Players")
local ReplicatedStorage = game:GetService("ReplicatedStorage")
local Teams = game:GetService("Teams")

local MAX_DIFFERENCE = 1 -- a team may have at most this many more players than the smallest
local SWITCH_COOLDOWN = 5 -- seconds between team changes for one player

local chooseTeam = Instance.new("RemoteEvent")
chooseTeam.Name = "ChooseTeam"
chooseTeam.Parent = ReplicatedStorage

local lastSwitch: { [Player]: number } = {}

-- Only teams with AutoAssignable off are offered in the picker
local function pickableTeams(): { Team }
	local list: { Team } = {}
	for _, child in Teams:GetChildren() do
		if child:IsA("Team") and not child.AutoAssignable then
			table.insert(list, child)
		end
	end
	return list
end

local function wouldUnbalance(team: Team, player: Player): boolean
	local smallest = math.huge
	for _, other in pickableTeams() do
		local count = #other:GetPlayers()
		if other == player.Team then
			count -= 1 -- the player is leaving that team
		end
		smallest = math.min(smallest, count)
	end
	return #team:GetPlayers() + 1 - smallest > MAX_DIFFERENCE
end

chooseTeam.OnServerEvent:Connect(function(player: Player, teamName: unknown)
	if typeof(teamName) ~= "string" then
		return
	end
	local team = Teams:FindFirstChild(teamName)
	if not (team and team:IsA("Team")) or team.AutoAssignable or player.Team == team then
		return
	end
	local now = os.clock()
	if now - (lastSwitch[player] or -math.huge) < SWITCH_COOLDOWN then
		return
	end
	if wouldUnbalance(team, player) then
		return
	end
	lastSwitch[player] = now
	player.Team = team
	player.Neutral = false
	-- Respawn so the player appears at a SpawnLocation with their team's TeamColor
	player:LoadCharacterAsync()
end)

Players.PlayerRemoving:Connect(function(player: Player)
	lastSwitch[player] = nil
end)

-- LocalScript in StarterPlayerScripts named TeamPicker.
-- One button per pickable team, in the team's colour, with a live player count.
local Players = game:GetService("Players")
local ReplicatedStorage = game:GetService("ReplicatedStorage")
local Teams = game:GetService("Teams")

local chooseTeam = ReplicatedStorage:WaitForChild("ChooseTeam") :: RemoteEvent
local player = Players.LocalPlayer

local gui = Instance.new("ScreenGui")
gui.Name = "TeamPicker"
gui.ResetOnSpawn = false
local bar = Instance.new("Frame")
bar.AnchorPoint = Vector2.new(0.5, 0)
bar.Position = UDim2.new(0.5, 0, 0, 12)
bar.Size = UDim2.fromOffset(0, 44)
bar.AutomaticSize = Enum.AutomaticSize.X
bar.BackgroundTransparency = 1
bar.Parent = gui
local layout = Instance.new("UIListLayout")
layout.FillDirection = Enum.FillDirection.Horizontal
layout.Padding = UDim.new(0, 8)
layout.Parent = bar
gui.Parent = player:WaitForChild("PlayerGui")

local buttons: { [Team]: TextButton } = {}

local function refresh()
	for team, button in buttons do
		local count = #team:GetPlayers()
		local mine = player.Team == team
		button.Text = string.format("%s%s (%d)", if mine then "> " else "", team.Name, count)
	end
end

local function addButton(team: Instance)
	if not team:IsA("Team") or team.AutoAssignable then
		return
	end
	local button = Instance.new("TextButton")
	button.Size = UDim2.fromOffset(150, 44)
	button.BackgroundColor3 = team.TeamColor.Color
	button.TextColor3 = Color3.new(1, 1, 1)
	button.Font = Enum.Font.GothamBold
	button.TextSize = 18
	local corner = Instance.new("UICorner")
	corner.CornerRadius = UDim.new(0, 10)
	corner.Parent = button
	button.Activated:Connect(function()
		chooseTeam:FireServer(team.Name)
	end)
	button.Parent = bar
	buttons[team] = button
	team.PlayerAdded:Connect(refresh)
	team.PlayerRemoved:Connect(refresh)
	refresh()
end

for _, team in Teams:GetChildren() do
	addButton(team)
end
Teams.ChildAdded:Connect(addButton)
player:GetPropertyChangedSignal("Team"):Connect(refresh)

-- LocalScript in StarterPlayerScripts named TeamDoors.
-- Parts tagged "TeamDoor" with a "Team" attribute let only that team walk through.
local CollectionService = game:GetService("CollectionService")
local Players = game:GetService("Players")

local player = Players.LocalPlayer

local function update(door: Instance)
	if not door:IsA("BasePart") then
		return
	end
	local allowed = door:GetAttribute("Team")
	local mine = player.Team and player.Team.Name
	-- Each device simulates its own character, so turning collision off here
	-- lets this player through while the door stays solid for everyone else
	door.CanCollide = not (mine ~= nil and allowed == mine)
end

local function updateAll()
	for _, door in CollectionService:GetTagged("TeamDoor") do
		update(door)
	end
end

CollectionService:GetInstanceAddedSignal("TeamDoor"):Connect(update)
player:GetPropertyChangedSignal("Team"):Connect(updateAll)
updateAll()