It's a real team effort. You find interesting ways to motivate members to help. We buy an epic pack every 300 because we've opened the guilds we cannot complete; so no point opening the even bigger ones right now. We raffle the epic packs or reward players for ranking high in other raids that don't have epic rewards. So for example, Boom Stick saved enough for 3 packs recently. I gave 2 out for help with Bogota 2 and 1 to someone that has been doing regular guild bar dailies.
Find what works for you, I've opened other raids by offering rare and uncommon packs as incentives. We bought an epic pack last week and gave it out, turned out to have a mythic Terminator. It was a good reward for a long time officer with no mythics. Have fun!