partner-contact/partner_group/models/res_partner.py

# Copyright 2018 Tecnativa - Sergio Teruel
# License AGPL-3.0 or later (https://www.gnu.org/licenses/agpl).
from odoo import fields, models


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

    group_id = fields.Many2one(
        comodel_name='res.partner',
        string='Group',
    )