Generate List of Missing Relationships
You have to define what you mean with "similar workgroups" and alter the relevant line but this is what I understand from your wording.

It will show all combinations of users from different workgroups (and similar in an arbitrary way, matching the first 3 characters of group name) that do not appear (in either order) in the cluserrx table:

; WITH UserCombinations AS
    ( SELECT r.logonid AS logonid,
             u.workgroup AS User_workgroup,
             p.logonid AS rxuser
             p.workgroup AS Provider_workgroup
      FROM cluser AS u
        JOIN cluser AS p
          ON  u.workgroup &lt; p.workgroup                     -- not identical workgroups
          AND LEFT(u.workgroup, 3) = LEFT(p.workgroup, 3)   -- but similar
    )
SELECT 
    uc.logonid,
    uc.User_workgroup,
    uc.rxuser
    uc.Provider_workgroup
FROM UserCombinations AS uc
WHERE NOT EXISTS
      ( SELECT *
        FROM cluserrx AS rx
        WHERE rx.logonid = uc.logonid  AND  rx.rxuser = uc.rxuser
           OR rx.logonid = uc.rxuser  AND  rx.rxuser = uc.logonid
      ) ;