Feature Request: Personal folders with a team

Currently a user can be given either read-only access or write+read. We would like to be able to have a `SevereOverfl0w,foo.accounts@team`. The idea being that accounts/hr would have personal data on me that colleagues should not see, but I am encrypting for members of that team (if it truly exists).

The issue does not mention a file, test, or entry point. Start by clarifying how personal data, team membership, and read/write permissions should interact, including the proposed account syntax. Done should include an agreed permission model and tests covering personal visibility and encryption for valid team members.

Written by the indexing model from the issue text.