This may sound selfish but I'd rather give it to core members only, everyweek. You don't really need too much people and weak people uasually will end up jump to another server or quit.
So my advice is give it to people who has high battle power, active in gnw, talkative, contributed the most to the group because that kind of player has high chance not end up quit the game. I've seen so many case like that, leader speard out even the rewards and eventually people quit the game so those rewards were wasted.