def _default_res_users(self):
    users_obj = self.env['res.users']
    users = []
    for user in users_obj.search([]):
        if user.has_group("custom_module.group_pos"):
            users.append(user.id)
    return users

managers = fields.Many2many('res.users', string="Users in charge",
                            default=_default_res_users)

Try This.