I don't know if 100kbps would be good enough to hold 600 users, it may be. Again as a rough estimate I'd say 300kbps to comfortably handle that kind of userload.
Actually having the space for the users does not require you to use up any bandwidth, but you'll most likely be charged for it (unless you're supporting it yourself). The actual clients connecting to the server and using it's resources will use bandwidth. If you have space for 600 users, but only have 12 connections, you're not going to need it all and you'll have plenty of space.
Regards,
