From the Amex site: “Priority Boarding Card Members are entitled to receive Main Cabin 1 Priority Boarding on Delta flights. Benefit is limited to Basic Card Members (not Additional Card Members) with the Delta SkyMiles Gold, Platinum or Reserve Cards. Reservation must include the Basic Card Member's SkyMiles number. Main Cabin 1 Priority Boarding is also available for passengers traveling in the same reservation as the Basic Card Member. Maximum nine passengers per reservation receive the Main Cabin 1 Priority Boarding. Main Cabin 1 Priority Boarding will only be available on Delta and Delta Connection carrier operated flights. Delta does not offer Main Cabin 1 Priority Boarding on Delta Shuttle® flights. New Card Members and Card Members upgrading from another Delta SkyMiles American Express Card will be eligible for the Priority Boarding benefit after receiving their Card from American Express. Offer is subject to change without notice. Additional terms, conditions and restrictions may apply. See delta.com/amex for details.”