User and UserGroup do not implement getField method, which in turn means they can not be used with translation and field helper, nor can the ez_field_value and ez_render_field be used directly on User/UserGroup (only on their 'content' property, accessed via magic method).

As they can be used however with ez_render_name, for example, and considering they do extend abstract 'content' class, it would make sense for them to behave in the same way the Content itself does.