Without something to test this against and using Dwain's tip on the row_number, the following may do what you want

;WITH NewIDAllRecords AS (

SELECT

jobDB.num AS JobNumber,

DateAdd(minute, 570, jobDB.entered) AS CreatedDate,

DateAdd(minute, 570, jobDB.closed) AS ClosedDate,

person.uniqueId AS UserID,

NEWID() AS Random

FROM jobDB

LEFT OUTER JOIN person ON jobDB.contactId=person.personID

WHERE jobDB.status=N'Closed'

AND DateAdd(minute, 570, jobDB.entered) >= DATEADD(d, -28, GETDATE())

AND DateAdd(minute, 570, jobDB.closed) >= DATEADD(d, -7, GETDATE())

AND person.uniqueId IS NOT NULL

),

SequenceAllUserRecords AS (

SELECT

ROW_NUMBER() OVER (PARTITION BY UserID ORDER BY Random) AS Seq,

JobNumber, CreatedDate, ClosedDate, UserID, Random

FROM NewIDAllRecords

)

SELECT TOP 50

JobNumber, CreatedDate, ClosedDate, UserID

FROM SequenceAllUserRecords

WHERE Seq = 1

ORDER BY Random