(I'm one of co-founders of Lookback)

As sksksk noticed there is an interface that comes with Lookback from which the user can turn on and off the recording. In newer versions of our SDK there is also a small indicator in the top menu so the user doesn't forget that the app is currently recording.

You create a special test build of the app that you send to users using something like TestFlight, at the end of the video, you see the lady turn off recording, so I assume you can turn it on through the same interface

A closer look at the lookback.io site makes me think the user will know they're being recorded.

> Send your app to users using TestFlight and ask them to start recording. You can also ask them to perform certain tasks you've prepared, typically sent via email.