So, my guild is constantly full and yet we still get a constant flow of people who want to join. While we can usually remove inactives from our ranks, it's becoming increasingly difficult. We've even thought of making a sub-guild to hold the overflow, but we've decided that's not a good idea and will probably separate the two groups.
So, why not have guild room expansions? We pay 500 energy to create the guild, perhaps we can somehow 'purchase' more space. Just an idea, and it would really help out.
I would recommend you to this thread: http://forums.spiralknights.com/en/node/15207