CWU: What is the "User Identification Number" required at E-filing > W2 E-Filing (EFW2) > Create File?
CWU Help explains this:
User ID — Enter the 8-character user ID assigned to the employee who is attesting to the accuracy of the file. This user ID is assigned by the Social Security Administration when the employee registers to use the SSA’s Business Services Online. To register, go to http://www.ssa.gov/bso/bsowelcome.htm.
(See Help > Contents > Contents (tab) > Electronic Filing > Uploading W-2s and W-3s > Creating the W-2/W-3 Upload File.)