How can I add grouping by category for employers in the standard employers view ?

Update

Right now I change the Search view adding this:

<filter string="Category" icon="terp-personal+" domain="[]" context="{'group_by':'category_ids'}"/>

And on clicking the button in the view I get this error:

File "/opt/openerp/openerp-server-6.0.4/bin/osv/orm.py", line 2170, in read_group
    assert not groupby or groupby in fields, "Fields in 'groupby' must appear in the list of fields to read (perhaps it's missing in the list view?)"
AssertionError: Fields in 'groupby' must appear in the list of fields to read (perhaps it's missing in the list view?)