old_contents: # -*- coding: utf-8 -*-
  from odoo import api, models
  
  
  class EventRegistration(models.Model):
      _inherit = "event.registration"
  
      @api.model
      def create(self, vals):
          res = super(EventRegistration, self).create(vals)
  
          if res.event_id.attendee_signup and res.attendee_partner_id:
              login = res.attendee_partner_id.email
              user = self.env['res.users']\
                         .search([('login', '=ilike', login)])
              if not user:
                  user = self.env['res.users']\
                             ._signup_create_user({
                                 'login': login,
                                 'partner_id': res.attendee_partner_id.id,
                             })
                  user.partner_id.signup_prepare()
  
          return res
  
new_contents: # -*- coding: utf-8 -*-
  from odoo import api, models
  
  
  class EventRegistration(models.Model):
      _inherit = "event.registration"
  
      @api.model
      def create(self, vals):
          res = super(EventRegistration, self).create(vals)
  
          if res.event_id.attendee_signup and res.attendee_partner_id:
              login = res.attendee_partner_id.email
              user = self.env['res.users']\
                         .search([('login', '=ilike', login)])
              if not user:
                  user = self.env['res.users']\
                             .sudo()\
                             ._signup_create_user({
                                 'login': login,
                                 'partner_id': res.attendee_partner_id.id,
                             })
                  user.partner_id.signup_prepare()
  
          return res
  
subject: allow create registration for Event Manager