Add the property's type "User or Group" (Nuxeo 7.2+)

> Add a "UserGroup" entry in the type dropdown
> when it's selected, open the advanced field configuration screen
> select the type string
> add a reference section
> add a dropdown reference choice with values None and "User or Group" and others existing
> automatically select the "User or Group" value
> add a reference parameter "type" which contains "Both User and Group", "Only Users" and "Only Groups"
> default to "Only Users"
> if the reference usergroup is selected and the type is not string, add an error (username and groupname are always strings)
> manage the corresponding XSD generation