Someone ask if you can add an Azure AD guest user to a distribution list. But in Exchange online you cannot find that guest user.
By default external Azure AD guest accounts are hidden from Global Address List. If you want to enable this, you can run the following command, in the AzureAD module.
Set-AzureADUser -ObjectId OF-THE-GUEST-ACCOUNT -ShowInAddressList $true
If the need is that the guest only needs to be added to a distribution list. Connect to the Exchange module and run the following command.
Add-DistributionGroupMember -identity firstname.lastname@example.org -Member Azure.AD.GuestUser@domain.com
The Azure AD guest user will be a guest mail user in EXO, but still hidden from the GAL.