share room calendar with people outside of the company

Hello,

we have customer which have M365 busines premium. We create him a room with calendar which he wants to share with people outside the company (not just view, but give them ability to read and as well right to create events to the calendar).

Is it even possible ? 

Or is there any other way to let external users use (read, write )the same calendar as the users in the company ? 

Thank you.