Which group should I add these users, who are getting the below error

Hi there,
    Few of the users whom i want to give read/write both access to my team projects and added them in contributor group and getting 'Object Refence is not set to an instance of an object error' and the below log entry is found in server event log. Should I add them in any other group
Thanks,
Anu


Event Type: Information
Event Source: VSTF Source Code Control
Event Category: None
Event ID: 3003
Date:  8/9/2005
Time:  1:15:55 PM
User:  N/A
Computer: TEST01
Description:
An exception has occurred in a Team Foundation component.
Technical Information (for the administrative staff):
Date (UTC): 8/9/2005 8:15:55 PM
Machine: TEST01
Application Domain: /LM/W3SVC/4/Root/SCC-4-127680055175451442
Assembly: Microsoft.VisualStudio.Hatteras.Server, Version=8.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a
Process Details:
  Process Name: w3wp
  Process Id: 2380
  Thread Id: 7
  Account name: ITC\bgordon

Detailed Message: An exception occurred in the Team Foundation Version Control System.
Exception Information:
  Exception Type: ResourceAccessException
  Exception Message: Access Denied: User ITC\bgordon needs the UseSystem global permission(s).
  Exception Stack Trace:
     at Microsoft.VisualStudio.Hatteras.Server.SecurityManager.CheckGlobalPermission(IPrincipal userPrincipal, GlobalPermissions permissionRequired)
   at Microsoft.VisualStudio.Hatteras.Server.Repository.QueryWorkspaces(String ownerName, String computer)

For more information, see Help and Support Center at http://go.microsoft.com/fwlink/events.asp.