Share office 365 calendar reading AND writing

Good mornin,

I am creating some calendars to share for the organization of appointments, but there is a need to share one of these calendars with external people, not only for reading but also for writing. The calendars I created are all on outlook, and in general our organization will use the web interface (to solve the OS difference problems). Does anybody knows a solution to do this?

I try to build an angular app (with graph api), but i still need an account to log-in.

Can I force the use of a specific account in my angular app?