From a Google FAQ:
If I share a collection with a mailing list, or set it to public (on the Web or in a domain for Enterprise users) will the collection automatically appear in other people's Documents List?
No, people in the group and those within your domain need to click the URL in the invitation you send for the shared collection to appear in their Documents List. When you set a collection to Public, everyone gets permission to access the collection. However, the collection appears in people's Documents List only after they've opened it by clicking a link sent via invite, email, or chat.
This, as it turn out, is a bad idea. As users join an organization or a group, or if they fail to click on the link sent to them, their document lists are not updated. The main feedback I get from users is that they feel that either they have done some thing wrong, or that there is something misconfiguration on the domain. I fail to see why Google have decided to do things this way, or why they do not provide a way to change this behavior.