[seabass-labrax]

My recollection is that, at the top-right corner of the screen, you can see the email addresses of the other people who are currently editing the document (but not those who did in the past). If they aren't logged in, you'll see them as names like 'Anonymous Aardvark' instead.

[mattzito]

That is inaccurate. You only see individuals with whom you have directly shared the document with and/or people in your domain (if a business/enterprise customer). For anything link shared, you see the anonymous animal names.