While planning your migration to Office 365, you have a couple options when provisioning Shared Mailboxes in Office 365.Microsoft does not charge you to create Shared Mailboxes in Office 365, if you do it right.Here are your options to create Shared mailboxes.
Typically, when planning a migration from a Lotus Notes or GroupWise messaging system, we will create/update all of our accounts in Active Directory and allow directory sync to populate them in Office 365.After doing so, you then assign a license/ mailbox to each of these accounts.When following these steps you essentially enable the ability to manage passwords (with ADFS) and the account state from Active Directory.This is a great process, for standard user objects; however, there are implications when following these steps for Shared Mailboxes.When you follow these same steps for a Shared Mailbox, you ultimately assign a license to the Shared resource, which you cannot undo.If you try to undo it, you delete the mailbox.So if you are OK with assigning a license and paying for that resource, then this approach will work well for you.
If you want to save money, you will need to create the resource directly in Office 365 using the Exchange Control Panel (ECP).You basically just create a user object in the ECP (not in the portal) and then run a PowerShell command to convert it to a Shared Mailbox. (e.g., set-mailbox “id” -type shared)
Now the object exists as a Shared mailbox and does not take up a license.I have noticed that some migration tools do not transfer ACL’s properly; by not matching to an AD account (I heard this may get resolved soon.)The other challenge with this approach is not being able to create a large number of Shared mailboxes to support your migration, in a short amount of time (unless you can convince Microsoft to release the license for these accounts, but keep the mailbox.)So there are definitely pros/ cons with each approach.Since a large number of Shared mailboxes can present a large monthly cost, I would highly recommend that you consider the approach to manually creating them in Office 365.