So you noticed the ‘All Users’, ‘Everyone’ and ‘Domain Users’ groups are missing in Office 365! That’s a pity if you have a mailbox that your whole organisation should be able to access, because Dynamic Distribution lists can’t be used as a security group.
At first, I thought this would be as simple as enabling ‘Dedicated Groups‘ in AzureAD for the tenant. But no, apparently builtin groups already exist, but are simply invisible in the Office 365 / Exchange Online interface.
So, I looked up the SID of the group I wanted to add here, and used Powershell to add the group to the mailbox’s ACL. Here’s an example, where we’re giving Everyone access to a Office 365 shared mailbox called ‘Public Calendar’:
Add-MailboxPermission -Identity "Public Calendar" -User S-1-1-0 -AccessRights FullAccess
Note: these permissions will be invisible in the web interface of Exchange Online, user Get-MailboxPermission to verify / view them.
Warning: obviously, you should never give ‘everyone’ Full Access permissions to anything.