SharePoint Online migrated to a new guest platform that provides support for more powerful guest scenarios. This article contains information for administrators about SharePoint Online guest platform migration behavior.
The effects of this migration are invisible to non-administrators. However, administrators may notice unexpected behavior in which all existing guest accounts appear as newly added in audit reports. This includes even those accounts that have had no invitations in a long time (perhaps years).
Consider the following example for the Contoso tenant:
A member of the Contoso tenant (contoso.onmicrosoft.com) issued an invitation to a guest user from Fabrikam: firstname.lastname@example.org. Notice that the invitation was accepted in the year 2014.
AcceptedAs (1): email@example.com
AcceptedOn (1): 2014-12-15T17:30:40Z
CreatedOn (1): 2014-12-15T17:30:40Z
CreationType (1): Invitation 
UserState (1): Accepted 
UserType (1): Guest 
Imagine that the SharePoint guest migration process for contoso.onmicrosoft.com occurred on July 25, 2016. An administrator might notice that firstname.lastname@example.org was created as a guest on that date, almost one and a half years after the latest invitation was accepted by email@example.com. The entry would be displayed as follows:
This behavior is by design. It is not the result of a system error or unauthorized activity.
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