What we do is we have a "member rank" which has no restrictions on it, a "raid ready" rank, which means they need to keep their gearing/enchanting/playing knowledge up to snuff, but no attendance requirements, and a "raider" rank. Raiders are required to attend 80% of all raids on a month by month basis, and to let us know on the forums or through signups when they won't make it. When it comes time for invites, raiders have priority over raid readies, who have priority over members. It's a strong incentive to become a raider, because people are rolled out almost every raid time, and members are almost always the first to be sent packing. So if you want to raid, you pretty much have to have raider rank, and show up 80%+ of the time and let us know when you won't make it.
Kathy, I said, "I'm lost" though I knew she was sleeping
I'm empty and aching and I don't know why
Counting the cars on the New Jersey Turnpike
They've all gone to look for America