Only known way to combine multiple fields in a predefined group_by is by defining a record for the ir.filters model.

This is also the only case in which Odoo combines two or more fields in a group_by in the UI by default that can be select by a user*. See for example https://github.com/odoo/odoo/blob/17.0/addons/account/report/account_invoice_report_view.xml#L61

To apply this approach, for example on the res.partner model, you would do the following:

view/res_partner_views.xml:

<?xml version="1.0" encoding="UTF-8" ?>
<odoo>
    <record id="filter_country_salesperson_group" model="ir.filters">
        <field name="name">By Country and Salesperson</field>
        <field name="model_id">res.partner</field>
        <field name="domain">[]</field>
        <field name="user_id" eval="False"/>
        <field name="context">{'group_by': ['country_id', 'user_id']}</field>
    </record>
</odoo>

Which would result in a filter stored as a favorite:

... and is essentially the same what Odoo does for example in the Invoicing App (technical name account):

A direct integration into the Group By-section does not seem to be possible.

If you want to prevent users from deleting (or changing) this particular filter (which could be easily done, since its a filter stored as a favorite just like any other) you could do a super().write() and super().unlink() extension checking for this particular record and raise an UserError when an attempt on changing or deleting is detected.

*) You can pass multiple pre-defined group_by filters when using a dedicated action, but according to your question and example source, this is not what you are looking for.