for 1), seems like you could do a proxy encryption solution.
edit: wrong way to phrase I think. What I mean to say is, have a message key to encrypt the body, but then rotate that when team membership changes, and "let them know" by updating a header that has the new message key encrypted using a key derived using each current member's public key.
edit: wrong way to phrase I think. What I mean to say is, have a message key to encrypt the body, but then rotate that when team membership changes, and "let them know" by updating a header that has the new message key encrypted using a key derived using each current member's public key.