Bandwidth requirement ?

That depends on several things:

• How active are the users: if they chat a lot or use IM only for important things
• How big are the rosters: if people have a lot of contacts, there will be a lot of presence messages
• If they have contacts at other servers
• If they participate on MUC rooms with a lot of activity

A user with little contacts that change presence every hour or two and chats only a little can consume around 0 bytes/s :) You can define the maximum incoming rate allowed for a user, and the default value is 1 Kb/s.

So, 100 users can consume from 0 Kbytes/s to a maximum of 100 Kbytes/s