subquery - how to refer to outer query value

The query below is working fine:

SELECT 
   tblCase.ID AS CaseID, tblCase.UserID AS MyCasesFilter, 
   tblGroupMembership.UserID AS GroupShareFilter, 
   tblDirectCaseSharing.ReceiverUserID AS DirectShareFilter, tblCase.EntryDate, 
   tblUser.LastName AS CaseAuthor, tblCase.Name AS CaseName, 
   COUNT(DISTINCT tblCaseImage.ID) AS TotalImages, 
   tblCaseType.Name AS CaseType, tblCase.SiteName, tblCase.Category, 
   tblCase.FollowUpDateTime, tblCase.Notes
FROM     
   tblDirectCaseSharing 
RIGHT OUTER JOIN
   tblCase 
INNER JOIN tblUser ON tblCase.UserID = tblUser.ID ON tblDirectCaseSharing.CaseID = tblCase.ID 
LEFT OUTER JOIN
   tblGroupMembership 
INNER JOIN
   tblGroupCase ON tblGroupMembership.GroupID = tblGroupCase.GroupID ON tblCase.ID = tblGroupCase.CaseID 
LEFT OUTER JOIN
   tblCaseType ON tblCase.CaseTypeID = tblCaseType.ID 
LEFT OUTER JOIN
   tblCaseImage ON tblCase.ID = tblCaseImage.CaseID
GROUP BY 
   tblCase.ID, tblCaseType.Name, tblCase.SiteName, tblCase.EntryDate, 
   tblCase.Category, tblCase.FollowUpDateTime, tblCase.Notes, tblCase.UserID, 
   tblGroupMembership.UserID, tblDirectCaseSharing.ReceiverUserID, 
   tblUser.LastName, tblCase.Name
HAVING 
   (tblCase.UserID = 1) 
   AND (tblGroupMembership.UserID = 2) 
   AND (tblDirectCaseSharing.ReceiverUserID = 3)
ORDER BY 
   tblCase.EntryDate DESC

I want to add an additional select column to the above result using a select subquery which is:

STUFF((
SELECT ', ' +tblGroup.Name  as [text()] 
FROM     tblCase INNER JOIN
                  tblGroupCase ON tblCase.ID = tblGroupCase.CaseID INNER JOIN
                  tblGroup ON tblGroupCase.GroupID = tblGroup.ID
WHERE tblCase.ID = ***
FOR XML PATH('')
),1,2,'')
AS ConcatGroupShares

The select subquery has a where clause and I need to get the tblcase.id which is the first column of the result set. how to i reference that value in the subquery.