I currently get a warning on startup stating that group "stb-admin" isn't found. The warning originates from /etc/dbus-1/system.d/system-tools-backends.conf file which belongs to app-admin/system-tools-backends
Wich version of gnome-system-tools do you use?
app-admin/system-tools-backends was 1.4.2 and app-admin/gnome-system-tools was not installable (bug 155376) Now as I am using experimental gnome overlay, app-admin/system-tools-backends-2.1.1 creates this user.
Added a -r1 ebuild to create the group.