For now, I'm leaving this here. However, technically, you're asking a non-ULX related question (Off-Topic).
ULX uses it's own user and group files, imported from the file you're currently asking about/working with that Gmod uses.
You'd simply add an extra set of brackets inside the main two "Users" brackets
http://wiki.garrysmod.com/?title=Player_GroupsDon't add yourself twice. It confuses some poorly made scripts. (Leave yourself in the superadmin part, not admin)
Code below leaves out all the // comments.
"Users"
{
"superadmin"
{
"Ryan" "blah"
}
"admin"
{
"blah1" "STEAM_blah1"
"blah2" "STEAM_blah2"
}
"donator"
{
"dblah1" "STEAM_blah1"
"dblah2" "STEAM_blah2"
}
}
{/code]