Hey Brett, for these case it is best to use Central Automation Studio
Basically CAS uses API to do various tasks. For your task, once you have added your account to CAS, Once all that is setup, you can use the Device Inventory page in CAS.
From there, restrict the search to just devices without a group (via a simple checkbox) and then filter and sort to find what they want. Then click the Move To Group button and select the group.
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