how i can control the number of computer allowed for a user to join the domain by using group policy?

Based on my knowledge, I don't think that you can increase the number of computers allowed for a user to join the domain via Group Policies. The number of workstations currently owned by a user is calculated by looking at the ms-DS-CreatorSID attribute of machine accounts. To modify Active Directory to allow more (or fewer) machine accounts on the domain, use the Adsiedit tool. You can refer to this Microsoft article: http://support.microsoft.com/kb/243327 This posting is provided "AS IS" with no warranties or guarantees , and confers no rights.