Table ULib.ucl stores group information (and many other things)
You could test existence of ranks in it if you wanted.
As for chacking if players are in groups, group checking can be done with or without ULib. We simply enhance some of the group check functions to include inheritance and can target features.
Then we go further and add a command or two that Gmod doesn't have.
Normal Gmod;
Player:IsUserGroup Player:IsSuperAdmin Player:IsAdminULib enhanced/additional;
See the commands at
http://ulyssesmod.net/docs/files/lua/ulib/shared/sh_ucl-lua.htmlAny mention of ucl is 'user control list', where we store group information and more about players on the server.