Easiest way would be to let Ulib rebuild the file.
Shut down server. Delete your gmod/data/ulib/groups.txt file.
Restart server. Should be rebuilt.
If not, well, here's all the stuff you'd have to add back to superadmin.
"superadmin"
{
"allow"
{
"ulx addgroup"
"ulx adduser"
"ulx adduserid"
"ulx cexec"
"ulx ent"
"ulx groupallow"
"ulx groupdeny"
"ulx hiddenecho"
"ulx logechocolorconsole"
"ulx logechocolordefault"
"ulx logechocoloreveryone"
"ulx logechocolormisc"
"ulx logechocolorplayer"
"ulx logechocolorplayerasgroup"
"ulx logechocolors"
"ulx logechocolorself"
"ulx logjoinleaveecho"
"ulx luarun"
"ulx rcon"
"ulx removegroup"
"ulx removeuser"
"ulx removeuserid"
"ulx renamegroup"
"ulx setgroupcantarget"
"ulx userallow"
"ulx userallowid"
"ulx userdeny"
"ulx userdenyid"
"xgui_gmsettings"
"xgui_managebans"
"xgui_managegroups"
"xgui_svsettings"
}
"inherit_from" "admin"
}
As you can see,
"xgui_gmsettings"
"xgui_managebans"
"xgui_managegroups"
"xgui_svsettings"
are probably the real ones you're looking for.