I've got a large Zotero library split into several large collections. I know that I'm able to share collections by creating a group in Zotero and then creating a collection within that group. Is it possible to share a pre-existing collection, though?

Alternatively, can I synchronize a collection in a group with a collection that is already in my local library? Sharing a collection within a group is great, but if that collection starts off as a copy of a collection I already have in my Zotero then I'm going to have to maintain two collections independently.