Bukkit

Now has an option for multiworld support! Allowing you to set specific limits in specific worlds.

For older users simply type in this line in the top of your config.yml

AllowMultiWorldConfig: true

After that you should be able to start the server and it run it perfectly fine.

Also for each of your groups you will need to define a default value per group, so if you don't define each world it will use the default, or the plugin will throw errors everytime someone changes a world or logs on