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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.