A child of the main attrib tag object.
<attrib Name="User" parent="anything" editable="0" visibleInEditor="1" default="" >
Unlike the System attributes, there are no subgroups, just a list of individual user-created attributes. Non-exhaustive test would suggest, the Tinderbox will allow groups within User attributes but that they can only be created in the TBX source and not via the UI. However, if created, they work, except in the Inspector (quickstamp pane).