By default domain users can only add 10 machines to the domain. It’s necessary to grant permissions to join/remove computers to domain for IT Support/Helpdesk users . This can be accomplished through the Delegation Control wizard
Steps to delegate control on the Computers Container:
1. Open the Active Directory Users and Computers snap-in.
2. Right-click the container under which you want the computers added, and press Delegate Control.
3. Press Next.
4. Press Add.
5. After adding all the users and/or groups, press Next.
6. Select Create custom task to delegate and press Next.

7. Select Only the following objects in the folder, check Computer objects, check the Create selected objects in this folder box, and press Next.

8. Check Reset Password, Read and write account restrictions, Validated write to DNS host name, Validated write to service principal name. Click Next and Finish.

9. Press Finish.
For more information about the minimum rights to join PC to the domain, please refer to:

Error message when non-administrator users who have been delegated control try to join computers to a Windows Server 2003-based or a Windows Server 2008-based domain controller: “Access is denied”
http://support.microsoft.com/kb/932455