I am working with a team and we are just getting into working with shared folders. Some things that would make this (great) feature better include:
Showing recently added references
As it is, users don't know when other users add a reference to a shared folder. It would be nice to have unread or something like that. Or some reference managers have a filter for 'added in the last week' and 'added in the last day'
Sorting/ filtering by user who added
Maybe related to the above, it would be nice to see the references sorted/ filtered by who added them
Parent folders as sum of child folders
As it is, parent folders hold their own references. It would be nice to have a type of folder that was a sum of all its children.
I like using tags and folders together in my personal library, but tags are note shared in shared. I think this has been brought up before.
This one has also been raised before... this is an important part of collaboration and a key missing feature.
Shared markup in Meta PDF
This one get a bit confusing when two users have the same PDF in their personal library and mark up that version. I think marking up the shared version results in sharing the markup, but this will get lost once someone puts the reference into their own library, which they have to do in order to cite with it in Google Docs (I think). I am still bit confused about that one and we have not tested it extensively.
Notwithstanding, Paperpile is the easiest sharing I have experienced after using several reference managers... nice work!