The typical bandwidth utilization per user approximately 150 kbps. The actual bandwidth consumed depends on the actual features used during the meeting.
Desktop Share with 24-bit color can consume higher bandwidth - especially on pages with rich color & graphics.
Please take a look at http://help.dimdim.com/activekb/questions/56/What+is+the+amount+of+Bandwidth+consumed+during+desktop+sharing%3F for details on Desktop sharing bandwidth.
Audio and Video typically consumes about 50 kbps for 1 AV stream. As there are more users with audio - the number of audio streams will increase and the bandwidth utilization will also rise - about 12 kbps per audio stream