Can you actually make that work for a collaborative app?

User A uploads file_a.txt and you want to encrypt it. What key do you use for that? It can't be attached to User A (e.g. their password or password hash) only otherwise User B won't be able to decrypt it. How would you set that up in a way that's still reasonable considering Basecamp use-case? (meaning: one of their goals is to make project collaboration simple)

I dunno, maybe generate a keypair to en/decrypt the content, then encrypt multiple copies of that keypair with per-user keys? A key-getting-key or something like that.

There's probably some huge issues there, but it's a start to answering the question.