Index: addons/openacademy/views/openacademy.xml
===================================================================
--- addons.orig/openacademy/views/openacademy.xml	2014-08-28 14:34:28.583004473 +0200
+++ addons/openacademy/views/openacademy.xml	2014-08-28 14:34:28.579004473 +0200
@@ -244,7 +244,7 @@
             <field name="arch" type="xml">
                 <form string="Add Attendees">
                     <group>
-                        <field name="session_id"/>
+                        <field name="session_ids"/>
                         <field name="attendee_ids"/>
                     </group>
                     <footer>
Index: addons/openacademy/wizard.py
===================================================================
--- addons.orig/openacademy/wizard.py	2014-08-28 14:34:28.583004473 +0200
+++ addons/openacademy/wizard.py	2014-08-28 14:34:28.579004473 +0200
@@ -5,14 +5,15 @@
 class Wizard(models.TransientModel):
     _name = 'openacademy.wizard'
 
-    def _default_session(self):
-        return self.env['openacademy.session'].browse(self._context.get('active_id'))
+    def _default_sessions(self):
+        return self.env['openacademy.session'].browse(self._context.get('active_ids'))
 
-    session_id = fields.Many2one('openacademy.session',
-        string="Session", required=True, default=_default_session)
+    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):
-        self.session_id.attendee_ids |= self.attendee_ids
+        for session in self.session_ids:
+            session.attendee_ids |= self.attendee_ids
         return {}