Private sharing between a team and an outsider

There is an option for sharing folders with two accounts like `user1,user2`. One of the uses for that is sharing data with an app or a service, like sharing user's website data with a hosting provider.

With the introduction of the teams feature there could be some use cases for app developers. Say, there could be a team's wiki service which data could be updated by any team (subteam) member. It could be really useful to be able to share a folder between a team and a service provider for such a case.

**Update:** team to team sharing would be also very useful for this case and for some other cases like cross-team data sharing, i.e. `customer.managers,vendor.clients`

The issue names no files, tests, or entry points to start from. Clarify the requirements for sharing between a team and an outsider and between two teams, then identify the existing folder-sharing and team-sharing entry points before defining completion criteria.

Written by the indexing model from the issue text.