I am setting up a task management app with the following structure
Users (Collection)
Organisations (Collection)
Tasks (Sub-Collection)
Categories (Sub-Collection)
The user as the ability to be in 1 or more 'Organisations' (this is captured in the User collection as a list of document references)
Once a user has logged in they should be able to see all tasks no matter what 'Organisation' they are a part of.