An example of Airbnb optimizing an invite email:
- Use inviter's name in the subject for social proof.
- Use big headline conveying clear value (get $40 on your first trip).
- Create urgency by setting a deadline, the date by which you can accept the invite.
- CTA button says "Accept invitation" instead of just "Sign up" creating more "sense of exclusivity".
- Inviter's name, social image, information on how long he's been a user - creates even more social proof.