Hello Alex,

For add group by, you need to add search view in the xml file.

For Ex :-

<record id="dmr_search_view" model="ir.ui.view">   

    <field name="name">dmr.search.view</field>   

    <field name="model">model.name</field>   

    <field name="arch" type="xml">       

        <search string="DMR">           

            <filter name="group_customer_id" string="Customer" icon="terp-partner" context="{'group_by':'field_name'}"/>       

        </search>   

    </field>

</record>

After adding search record, you need to add search_view_id in action.

For Ex :-

In action,

<field name="search_view_id" ref="dmr_search_view"/>

Hope it works for you.

Thanks,