Anyone know anything about the ChatSystem? I don't understand it at all..

Make a ServerScript in ServerScriptService and put this code in (change values as nessacary shown in the comments!)

local ChatService = require(game:GetService("ServerScriptService"):WaitForChild("ChatServiceRunner"):WaitForChild("ChatService"))
local groupId = 000000 -- Group ID here

game.Players.PlayerAdded:Connect(function(player)
	while not ChatService:GetSpeaker(player.Name) do wait() end -- Wait till the player's speaker (thing that allows it to chat) exists
	local speaker = ChatService:GetSpeaker(player.Name)
	if player:GetRoleInGroup(groupId) == "Guest" then return end -- Remove this if you want it to do say [Guest] if they player isn't in the group
	speaker:SetExtraData("Tags", {{TagText = player:GetRoleInGroup(groupId), TagColor = Color3.fromRGB(0, 255, 0)}})
	-- Change the values in Color3.fromRGB() for a different tag color
end)

For the whisper chat (do this pre-runtime)(anytime i bold a name you must write it just like that):

1. Insert a Folder into Chat called "ChatModules"
2. Insert a BoolValue into **ChatModules** called "InsertDefaultModules" and set it's value to true
3. Insert a ModuleScript into ChatModules and put this in the module (make sure to make the remote event that the script references!)

local sendPopup = game.ReplicatedStorage.SendWhisperChatPopup -- Reference a remote event
local TextService = game:GetService("TextService") -- Filter messages so they have no swears

local function Run(ChatService)
	local function filterMessage(message, speaker, reciever)
		local filteringResult = TextService:FilterStringAsync(message, speaker.UserId)
		if not game.Chat:CanUserChatAsync(speaker.UserId) or not game.Chat:CanUserChatAsync(reciever.UserId) or not game.Chat:CanUsersChatAsync(speaker.UserId, reciever.UserId) then
			-- Player(s) cannot chat, send back a DONT SEND MESSAGE prompt
			return 0
		end
		return filteringResult:GetChatForUserAsync(reciever.UserId)
	end
	
	local function whisperCheck(speakerName, message, channelName)
		if string.sub(channelName, 1, 2) == "To" then -- If whisper channel then
			local otherPlayer = game.Players:FindFirstChild(string.sub(channelName, 4)) -- Get the other player
			local speakerPlayer = game.Players:FindFirstChild(speakerName) -- Get the player who said something
			local newMessage
			local succ, err = pcall(function() newMessage = filterMessage(message, speakerPlayer, otherPlayer) end)
			if newMessage == 0 then
				-- Player can't chat
				sendPopup:FireClient(speakerPlayer, "Note", "One of you can't chat!")
				return true
			end
			if err then
				sendPopup:FireClient(speakerPlayer, "Filtering failure", err)
				return true
			end
			-- We can send message
			sendPopup:FireClient(otherPlayer, speakerName, newMessage)
			return true
		end
		return false
	end
 
	ChatService:RegisterProcessCommandsFunction("whisperCheck", whisperCheck)
end
 
return Run

1. Next, make a ScreenGui in StarterGui
2. Insert a LocalScript into the ScreenGui and put this in it (remember to reference the remote event you made earlier where you need to!)

local inSession = false -- if another chat is on screen
local START_POS = UDim2.new(0, -300, 1, 0)
local END_POS = UDim2.new(0, 0, 1, 0)
local SIZE = UDim2.new(0, 200, 0, 50)
local ANCHOR_POINT = Vector2.new(0,1)
local TIME = 0.2
local doneEvent = Instance.new("BindableEvent", script.Parent)

local function showLabel(speakerName, message)
	if not inSession then
		inSession = true
		local label = Instance.new("TextLabel", script.Parent)
		label.Name = "WhisperBox"
		label.BackgroundColor3 = Color3.fromRGB(0,0,0)
		label.BackgroundTransparency = 0.5
		label.BorderSizePixel = 0
		label.Font = Enum.Font.SciFi
		label.TextScaled = true
		label.TextColor3 = Color3.fromRGB(255,255,255)
		label.AnchorPoint = ANCHOR_POINT
		label.Position = START_POS
		label.Size = SIZE
		label.Text = speakerName..": "..message
		label:TweenPosition(END_POS, Enum.EasingDirection.Out, Enum.EasingStyle.Quad, TIME, true)
		wait(5 + TIME)
		label:TweenPosition(START_POS, Enum.EasingDirection.Out, Enum.EasingStyle.Quad, TIME, true)
		wait(TIME)
		label:Destroy()
		label = nil
		inSession = false
		doneEvent:Fire()
	else
		doneEvent.Event:Wait()
		showLabel(speakerName, message)
	end
end

game.ReplicatedStorage.SendWhisperChatPopup.OnClientEvent:Connect(showLabel)

I tested it and it works!