partner-contact/partner_company_group/models/sale_order.py

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

from odoo import models, fields


class SaleOrder(models.Model):
    _inherit = 'sale.order'

    company_group_id = fields.Many2one(
        related='partner_id.company_group_id',
        store=True
    )