@Ramesh Rao, Thanks for posting in Q&A.
Currently, to enroll existing Azure AD joined devices to Intune, the options we can try are as below which mentioned by the article Rudy provided:
Option 1: Group Policy. (Local Group Policy)
Option 2. Registry:
Option 3. PowerShell
If there are a lot such devices, I think PowerShell script maybe more suitable for you.
Meanwhile, please ensure the Microsoft Intune licenses and Azure AD Premium license are assigned. And the MDM user scope under Automatic enrollment is set as all.
In General, there are many enrollment methods for windows device. If you have any new device which is not Azure AD joined yet, you can choose one method according to your situation. Here is a link with the details of windows enrollment methods for your reference:
https://learn.microsoft.com/en-us/mem/intune/fundamentals/deployment-guide-enrollment-windows
Hope it can help.
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