Error User not found at the time of new group creating

Question

Dear All, 
It's quit urgent i am getting an error while creating the new group in sharepoint
System.ArgumentException: User cannot be found.   
 at Microsoft.SharePoint.SPList.GetItemById(String strId, Int32 id, String strRootFolder, Boolean cacheRowsetAndId, String strViewFields, Boolean bDatesInUtc)    
 at Microsoft.SharePoint.SPList.GetItemByIdSelectedFields(Int32 id, String[] fields)    
 at Microsoft.SharePoint.ApplicationPages.PeoplePage.GetCurrentGroupListItem(String field)    
 at Microsoft.SharePoint.ApplicationPages.PeoplePage.OnLoad(EventArgs e)    
 at System.Web.UI.Control.LoadRecursive()    
 at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)
Just for an update i found another thing the same error arrive when i try to change the following 
1) Assign new primary administrator through the specific site collection 
2) Assign site collection administrator through site settings 
I already test the thing with new web application & site collection in other site collection every thing works fine ? 
Please if any one know answer it ASAP 
RB

Answer 1

The problem is likely in the user info list on the site collection.  Each site collection has its own user info list, and people are added to this with an ID that is unique to that person on that site collection.  Go to the following url of your site
collection and check the list.
http://sitecollection/_catalogs/users/detail.aspx
The list of
users of the
site collection _catalogs/users/detail.aspx also
be seen as
/_layouts/people.aspx?MembershipGroupId=0
You can test
whether the
user is
there?
You may also restart the User Profile Services and see id this is resolved.