While this seems at least one way to share contents of a collection without duplicating it, I'm not completely sure if there are any caveats I'm currently not aware of.
Definitely not. So I got this wrong. Judging from the UX, I expected a different behaviour. It would be more intuitive if the key granted permission only to the the collection to the feed of which I want to subscribe. Thanks for clarifying!
Upgrade Storage
1) Select of of your collections, click on the three dots in the upper row and then choose "Subscribe to Feed.
2) You're now asked to generate a private key (choose an appropriate name and define permissions).
3) After clicking "Save key" the (atom) feed is downloaded
4) the first "id" element in the "top.atom" file is the URL of the feed (e.g. `http://zotero.org/users/0123456/collections/ABCABC18/items/top?direction=asc&sort=dateModified`to which you have to attach your private key by adding `&key=WHATEVERYOURKEYIS`
5) the key appears to be strangely hard to access. Go to the key overview page (https://www.zotero.org/settings/keys) and find the key behind the "edit key" or "delete key" links (e.g. `https://www.zotero.org/settings/keys/edit/WHATEVERYOURKEYIS`).
With the complete URL (e.g. `http://zotero.org/users/0123456/collections/ABCABC18/items/top?direction=asc&sort=dateModified&key=WHATEVERYOURKEYIS`) you can subscribe to any collection via your feed reader.
While this seems at least one way to share contents of a collection without duplicating it, I'm not completely sure if there are any caveats I'm currently not aware of.