If you'd like someone else to know when you derail, head to the goal's page and click on the Settings tab under the goal. Scroll down until you get to the Supporters header. You can put their email in the box here and then click Add supporter. This will send them an email confirming they want to be signed up to that, and once you're all set, we'll send them a copy of your legitimacy check email when you derail on this goal. You'll need to set it separately for each goal.
A word of warning: please make sure the people you add like this are okay with getting emails from Beeminder beforehand! If they get an email they don't recognize or expect and they mark it as spam, that trains spam filters to see Beeminder emails as spam, which is bad for us (and if they just spambox our email, that defeats the point for you as well!).