Please consider leveraging Microsoft To-Do https://todo.microsoft.com/en-us
Thanks for your feedback. We are currently thinking about this. Can you please let us know what scenarios your organization uses shared mailboxes for and what options do you use. This will help us.
We create a shared mailbox for every client of a certain type. There are documents that are also assigned to these clients so it would be useful to keep everything in one place rather than keeping documents in a separate file structure.
We have announced that Group Owners will be able to leverage existing groups to drive membership in Office 365 Groups.
How does this basic functionality not exist yet? It is fundamental!
Even setting up a basic SharePoint environment with a few sites, I have my users from different guest organisations in security groups - I shouldn't have to manually add every individual to every O365 Group site.
This is currently stated as "In the Plans"... this should be a top priority!
Why is this still an issue??? Microsoft get your sh*t together.