An update is being rolled out to let you manage allowed/blocked domains for guest access to Office 365 Groups.
After allowing guest access to Groups, Microsoft now helps securing this access by allowing administrators to define a list of allowed/blocked domains.
This feature is not available (yet?) using the Office 365 administration portal but with PowerShell.
This functionality is using Azure AD policy feature
The PowerShell command to set the domain allow/block list for Office 365 Groups guest access is using the preview modules of Azure Active Directory PowerShell modules.
Once you have the required module installed, you can use the script available https://technet.microsoft.com/library/a86bb46f-0e5b-43a3-b6ef-7394f344a8da#bkmk_script to manage the domains list.
Once you have saved the script you can then use it to add/update/remove/import the domains list
Unfortunately there is not (yet?) a way to get the existing list or remove one domain; if you want to remove one domain you need to overwrite the list with the domain(s) you want to remove not included
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