You can now see how many people are in chat at anytime. The figure is displayed in the header.

This is part of the phpBB chat system I wrote awhile back, again the source code is free for counting active users:

(Yes, you have to cycle through each result, not possible to num_rows with the phpBB dbal sadly)

It's worth noting this function is cached by a minute to reduce load on the server. So results may be slightly off.