Can employers reimburse Medicare-eligible employees for the cost of Medicare Part B and/or a Medicare supplement?

This question is answered in IRS Notice 2015-17: https://www.irs.gov/pub/irs-drop/n-15-17.pdf

Employers with fewer than 20 employees (groups where Medicare is primary) CAN reimburse eligible employees for Medicare Part B, Part D, and Supplement premiums if they meet certain conditions.

Employers with 20 or more employees are subject to the Medicare Secondary Payer rules and cannot reimburse employees for Medicare premiums.