partner-contact/partner_company_group/models/res_partner.py

# Copyright 2019 Camptocamp SA
# License AGPL-3.0 or later (http://www.gnu.org/licenses/agpl.html).

from odoo import models, fields


class Contact(models.Model):
    _inherit = 'res.partner'

    company_group_id = fields.Many2one(
        'res.partner',
        'Company group',
        domain=[('is_company', '=', True)]
    )

    def _commercial_fields(self):
        return super(Contact, self)._commercial_fields() + ['company_group_id']