I remember one time, oh, 2-3 years ago, some discussion with Megiddo about "user" group and it not showing in "ulx who" or some other group checking code or some such I was working on.
At the time, I think my code kept showing as nil for the group 'user', and also, Megiddo explained that if they don't show as a group, well, they're just general 'users'.
I could imagine that being the case for XGUI too, but, its been many years, and Megiddo/you may have made changes to that.
If "user" isn't showing as an autodetect, you may need to force it for allowing folks to edit it.