Mail Enabled External Users - Allow UserType=Member to be Mail Enabled - Allow Inviter to Control Mail Enable/ShowInAddressList on invite
Consider the following scenario. You have a sister company or other company with their own AzureAD tenant, for which you want your users to be able to collaborate. You invite those users to your AzureAD tenant. Depending on how you do so, those external users may be Users or may be Mail Enabled Users.
These External Users may now collaborate with your users using SharePoint or other AzureAD connected business apps, but if you're users are regularly collaborating with these external users, wouldn't it be great if you could make it easy for your users to find them in the Exchange Global Address List?
In the past you would need to also create Contacts in Exchange Online in order for your users to find these objects in and Exchange Address Book.
By default Mail Enabled Users are hidden from the Exchange Global Address List but you can change the attribute ShowInAddressList to True and now your External User has both the functionality of a User AND a Contact.
Now, consider these External Users are actually payrolled staff who happen to use another AzureAD Tenant for any number of reasons. Maybe they are part of the same company but have a tenant registered in a different geography. Maybe they are a sister company or users from a company you acquired. ...or maybe they are just users of a very close business partner who you want to have full access to your tenant. In this case you want to provision them as UserType Member instead of UserType Guest.
Invitations for External Guests are provisioned as "Mail Enabled Guests" but invitations for External Members are provisioned only as "Users". Using Set-ADUser you can change the UserType to Guest, which will automatically Mail Enable the user, and then you can change it back to UserType Member and it will forever remain a Mail Enabled user.
Why not give the inviter explicit control of this behaviour on invite?
This is powerful stuff here. This makes it very easy for a company to manage users in multiple tenants as if they are one. From a user perspective they can easily find these external users in Exchange/Outlook/SharePoint, etc.
Microsoft should enable portal.azure.com and Azure AD PowerShell to explicitly set Mail Enabled to True or False for both Member and Guest invitations, and also set the ShowInAddressList attribute to True or False during invitation rather than having to invite all as Guest and change them to Member and set the ShowInAddressList attribute after invitation.
Luke Smith commented
Would be good to see this or is it possible to create and address book for guest users?