With 5.6 the collections API has been entirely re-written. This brings some conceptual changes and larger feature set to collections.
The main difference between the old and new implementation is that a user is no longer created for external shares. Instead the share impersonates the owner of the collections when the assets are accessed via a sharing token (link or e-mail).
On top of that it adds the following features
More sharing options
Link (External)
Groups (Internal)
Expiration date for all shares
Revoking a share
Overview of who it is shared with
It is no longer possible to remove of a shared collection from your collections page.
For more information and a migration path, see the following documentation