This depends a lot on your deployment (unique IP addresses? Do you use authentication? etc.). Depending on that, you can for example use some CLI magic using netstat/lsof, awk, grep, uniq etc.
But this will only work if a single user corresponds to a single IP address.
I would be interested in this too
Either by users authenticated by NTLM or by IP addresses.
Also, is there a way in the report builder to see how many users are on at a previous time. For instance, a bar graph that would more than likely show that 12pm and 1pm would have a higher number of users than 9am.
We use Web Reporter and a very simple filter to get a unique count based on NTLM user ID for the time frame we specify.