If you want to register attendees to multiple sessions follow the below code in your Odoo module.

                <group>
                    <field name="session_ids">
                    <field name="attendee_ids">
                </field></field></group>
                <footer>
                    <button name="subscribe" type="object"

openacademy/wizard.py

class Wizard(models.TransientModel):
    _name = 'openacademy.wizard'

    def _default_sessions(self):
        return self.env['openacademy.session'].browse(self._context.get('active_ids'))

    session_ids = fields.Many2many('openacademy.session',
        string="Sessions", required=True, default=_default_sessions)
    attendee_ids = fields.Many2many('res.partner', string="Attendees")

    @api.multi
    def subscribe(self):
        for session in self.session_ids:
            session.attendee_ids |= self.attendee_ids
        return {}

Note-Modify the wizard model so that attendees can be registered to multiple sessions.