SharePoint Online and OneDrive for Business are making increasing use of Azure B2B Collaboration for sharing, which means that Office 365 tenants will have more guest user accounts. Using Azure B2B Collaboration is a reasonable approach because it's secure and works well, but it does mean that tenant admins have some extra work to do to keep an eye on those guest accounts.
https://www.petri.com/sharepoint-onedrive-azure-b2b-collaboration
