Hi, @Glenn Maxwell
Thank you for posting in Microsoft Q&A forum.
We can use PowerShell command Add-CMDeviceCollectionDirectMembershipRule easily bulk add multiple devices to SCCM device collection.
Here is the details you may refer:
https://www.prajwaldesai.com/add-multiple-devices-to-sccm-collection-using-powershell/
(Please note: Information posted in the given link is hosted by a third party. Microsoft does not guarantee the accuracy and effectiveness of information.)
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