How would I make a customer not be able to see a surface gui?

Your code:

game.Players.PlayerAdded:Connect(function(player)
    if player:IsInGroup(3639089) and player:GetRankInGroup(3639089) >= 3 then
        player.PlayerGui.SurfaceGui.Enabled = true
    end
end

Your problem:

You either have not used a local script or it is because you put end end instead of end end)

How to fix the problem:

use a local script and don't forget to say end), not all cases use end

Fixed code: (USE A local script)

game.Players.PlayerAdded:Connect(function(player)
    if player:IsInGroup(3639089) and player:GetRankInGroup(3639089) >= 3 then
        player.PlayerGui.SurfaceGui.Enabled = true
    end
end) --Here is your problem the script will not work without it! 

More detail:

When using a function you always need to end with end), end will just break the script and if using PlayerGui you must use a local script!

Accept and upvote if this helps!