Anyone played Dark Orbit? That game also has guilds (called clans).
Once a day, the clan deducts 2-5% of your resources (goes to the treasury).
You might start thinking that sucked, but you can also transfer resources to other players.
So if I had 10k of each resources every day, then the guild will automatically deduct 300 of each of my resources, all in all 1,200 (300 wood, stone, iron and gold)
So once the resources go high in the guild, the leader and vice-leader can give resources to other fellow guild members. But of course, you'd have to pick you vice-leader properly.
You can also add resources to the guild (not from auto-deduction), but there should be a limited cap, like maybe 25k a day from each member.
So with this, people would really want to pick the good guilds.