Calendars that are synced locally from the stock iOS Calendar app do not yet have the ability to add invitees, but will soon with a new version Boxer.
To add an invitee to an Exchange calendar event:
1. Open an existing event or tap on the "+" sign to create a new event.
2. Tap on "Calendar" to make sure you have your Exchange calendar selected.
3. Then, in the "new event" screen, you should now see the option for "Invitees" below "Calendar". Tap on "Invitees" to add a new invitee.