I read that the following works:

1. Sign up for the Android beta programme: g.co/androidbeta

2. Enroll your Nexus

3. Unenroll after you have been updated to Android 7

Make sure you are actually on Android 7 before unenrolling. Going out of a regular beta will wipe your phone!

Cheers

Edit: Copy/Paste sandGorgon’s warning from below.