How to autobalance teams when using an gui to do so?

This is actually quite simple. To start off let's see how many players are on each team:

local police = game.Teams.Police
local prisoner = game.Teams.Prisoner

local numpolice = #police:GetPlayers()
local numprisoner = #prisoner:GetPlayers()

The :GetPlayers() function gives us a table, a group of objects. When we put a # in front of a table, it will give us the number of objects in the table. In this case, our number of objects is the number of players on a team

Now, depending on how many players are on each team, let's see if our player can join a team.

game.Players.LocalPlayer.PlayerGui.ScreenGui.Teams.Police.MouseButton1Click:Connect(function()
    if numpolice <= numprisoner then
        -- We allow them to join the team

    elseif numpolice > numprisoner then
    -- There are too many players, you could possibly put a GUI here that says that team has too many players

    end
end)

game.Players.LocalPlayer.PlayerGui.ScreenGui.Teams.Prisoner.MouseButton1Click:Connect(function()

    if numprisoner <= numpolice then
        -- Allow them to join the team

    elseif numprisoner > numpolice then
        -- Again, the teams are too full, so we can tell them by displaying a GUI
    end
end)

So to recap what we just did, we set up a function to connect when the TextButton, or the team join button, is clicked. Inside this function, set up some things to determine whether they can join a team or not. First, we asked the script to figure out this: Are the number of police less than or equal to the prisoners? If so, we can allow them to join the team. Otherwise, they can't join it. We did the same with the prisoners.

I hope this helped answer your question!