Would be pretty confident that they would be able to allocate tickets prior to general public sales. The info below is from the website and website lists the conditions:
Grand Final Guarantee
Legends, Trident and Redlegs members have guaranteed access to purchase a Grand Final ticket if Melbourne is competing, as part of their membership package.
Demon Seats, MFC/MCC and Red and Blue Members can guarantee access to purchase finals tickets and a Grand Final ticket, should the Demons make it in 2017, by purchasing the Grand Final Guarantee add on with their membership.
GF Guarantee Adult
GF Guarantee Concession
GF Guarantee Junior
GF Guarantee Country
If you are a demon seat home member, it would be 50 bucks cheaper to upgrade to a redlegs home membership rather than take out a GFG.