To put it lightly, the Amara platform needs work. If you try to sign in with both Google and Facebook and the same e-mail address, you end up creating two separate accounts that are impossible to link/merge. Never mind that people have multiple email addresses and phone numbers and YouTube accounts in real life.

If it has existed for years, why wasn't it used to save the UC Berkeley library videos?

I'm thinking more of a Google Docs style interface, where you can make a lightweight edit (only one or two lines of captions) and someone else can concurrently time and edit other lines.