old_contents: # Copyright 2019 Camptocamp SA
  # License AGPL-3.0 or later (http://www.gnu.org/licenses/agpl.html).
  
  from odoo import fields, models
  
  
  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()._commercial_fields() + ["company_group_id"]
  
new_contents: # Copyright 2019 Camptocamp SA
  # License AGPL-3.0 or later (http://www.gnu.org/licenses/agpl.html).
  
  from odoo import fields, models
  
  
  class Contact(models.Model):
      _inherit = "res.partner"
  
      company_group_id = fields.Many2one(
          "res.partner", "Company group", domain=[("is_company", "=", True)]
      )
      company_group_member_ids = fields.One2many(
          comodel_name="res.partner",
          inverse_name="company_group_id",
          string="Company group members",
      )
  
      def _commercial_fields(self):
          return super()._commercial_fields() + ["company_group_id"]
  
subject: Add one2many counterpart for company_group_id TT34815