How do I make a group rank GUI appear above players head?

It's a simple script, you need to know how to use Billboard Gui and the script.

Put scriptA inside Workspace, scriptB inside Billboard Gui and Billboard Gui inside Lighting.

Example scriptA inside Workspace:

game.Workspace.ChildAdded:connect(function(rplr)
	game.Lighting.RankTag:Clone().Parent = rplr.Head		
	-- Moving the tag into player's head
end)

Example scriptB inside Billboard Gui:

script.Parent.TextLabel.Text = game.Players.LocalPlayer:GetRoleInGroup()	
-- Your group ID

There's several tutorials in Youtube on how to make the Tag.

I hope this helps you out! :)