Google Tasks API - add a task to another user

Question

Google

There is a certain web application, it has its own client_key client_id. Using Google OAuth authentication, I log in and register (using google mail as an identifier) ​​in my application.
Further, this user can add events and tasks to the Google calendar (the application has its own tasks and events - duplication in Google is a “gadget”), the task is to add tasks to another user from our application.

Has anyone done something similar?

You can probably store tokens from Google in your system, and using the required token from the user to add a task to him, but how safe will it be and whether Google will allow it to do this (store offline tokens).
Another option is to delay adding such a task to the queue until the desired user enters the system.
And another option is to store the Google password at home and use the old google client api to get a token and work with the user offline.

The ideal would be a hypothetical option in which the user to whom we want to send tasks somehow shared / opened access to his calendar to our web application ... And after that, Google quietly allowed manipulating his data in api requests ...