overhead billboard gui only appears on 1 player?

Your first variable is saying you have only one copy of the namegui so the way to fix that is to put that between line 5 and 6 like this!

game.Players.PlayerAdded:Connect(function(p)
    p.CharacterAdded:Connect(function(c)
        if c ~= nil and c:FindFirstChild('Head') then -- Just to make sure the character has loaded completely
            local name = game.ServerStorage.namegui:Clone()
            local hed = c.Head
            name.Parent = c.Head
            hed.namegui.TextButton.Text = p.Name 
        end
    end)
end)

What you are doing in your code is that you are cloning it only once, when the script first runs. What you have to do is clone it every single time a player joins.

Here's your new code:

local name = game.ServerStorage.namegui:Clone()

game.Players.PlayerAdded:Connect(function(p)
    p.CharacterAdded:Connect(function(c)
        local head = c.Head
        name.Parent = head
        head.namegui.TextButton.Text = p.Name
    end)
end)