@Chloe Hall, Thanks for posting in Q&A.
From your description, I know you want to add an enrolled user to another group in the MDM user scope and want to know if there will be any problem in enrollment. If there is any misunderstanding, feel free to let me know.
Based as I know, if we add a user who is already enrolled to Intune to another group and want to re-enroll it, it won't be a problem but only if you unenroll old device from Intune and remove the previously enrolled device from Microsoft Intune portal and Microsoft Entra portal and add user group to the MDM user scope which you want to auto-enroll. Otherwise, the user will not be able to enroll in Intune.
https://zcusa.951200.xyz/en-us/mem/intune/user-help/unenroll-your-device-from-intune-windows
So, it will not impact the enrollment after switching the user to another user group if you have added the user group to MDM user scope.
Hope it will help.
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