SP To Perform Query Based On Multiple Rows From Another Query's Result Set

I have two tables .. in one (containing user data, lets call it u).
The important fields are:
u.userName, u.userID (uniqueidentifier) and u.workgroupID (uniqueidentifier)

The second table (w) has fields
w.delegateID (uniqueidentifier), w.workgroupID (uniqueidentifier)
The SP takes the delegateID and I want to gather all the people from table u where any of the workgroupID's for that delegate match in w.  one delegateID may be tied to multiple workgroupID's.
 I know I can create a temporary table (@wgs) and do a:
 INSERT INTO @wgs SELECT workgroupID from w WHERE delegateID = @delegateID
that creates a result set with all the workgroupID's .. this may be one, none or multiple
I then want to get all u.userName, u.userID FROM u WHERE u.workgroupID
This query works on an individual workgroupID (using another temp table, @users to aggregate the results was my thought, so that's included)

         INSERT INTO @users
            SELECT u.userName,u.userID
                FROM  tableU u
                LEFT JOIN tableW w ON w.workgroupID = u.workgroupID
                WHERE u.workgroupID = @workGroupID
I'm trying to avoid looping or using a CURSOR for the performance hit (had to kick the development server after one of the cursor attempts yesterday)

Essentially what I'm after is:
             SELECT u.userName,u.userID

                FROM  tableU u

                LEFT JOIN tableW w ON w.workgroupID = u.workgroupID

                WHERE u.workgroupID = (SELECT workgroupID from w WHERE delegateID = @delegateID)
... but that syntax does not work and I haven't found another work around yet.
TIA!