What organization do the users belong to? The product team search will only search for users that are part of the product's org.
And the "Username is not Unique" error would occur if the original users still existed in the DB. As Syed asked above, did you delete the users from both Windchill and the Directory server? If the users were only deleted from the directory server, then they still exist in the DB. Use the principal administrator to find and delete the users. Then you should be able to recreate them.