I would like to recommend programmes to other individual members.

Currently I set up a collection for them and have them subscribe to it.

It would be nice if I every member had an "inbox" collection that anyone else could add to. (Other than that it works just like a normal collection, for example only the owner can delete programmes again).

Of course, this is a potential spam hazard, so there may need to be some access control later, such as a friend system (only friends can add).

An inbox collection could be a new type of collection or (probably easier) just a flag that can be enabled on existing collections.