As a Platinum Medallion, you can check up to 3 bags, 70 pounds each within most domestic markets. Also, up to 8 additional companions traveling on the same flights to the same final destination enjoy the same baggage allowance and priority baggage handling as the Medallion customer.

If booked in the same PNR, bag fee waivers will be automatic and simple. If booked in separate PNRs, advise the Bag Drop agent that your companions are traveling with you to the same destination on the same flights and the agent will need to manually waive applicable fees. (There's nothing that can be done before arriving at the airport to avoid speaking directly with a ticket counter agent to waive bag fees for companions.)

It's unfortunate if Bag Drop agents don't waive bags for Medallions' companions. There's actually a specific and conveniently located bag fee waiver code for Sky Priority companions in the Bag Drop software, so ask for a Red Coat if someone denies the bag fee waiver. I really don't think it will be a problem, though.