Its only a suggestion, on another forums that I have account, I usually see the number of online users seeing the same forum. On my opinion it would be good for users that make questions to see the number of online users that could help him quickly. I take a look on search and didnt see discussions about this before on meta.
Do you have any reason to believe questions aren't being answered at the moment? In other words, displaying who's online won't raise your chances of getting a (good) answer.
Besides, you'd often want somebody knowledgeable to post an answer, if none of them are online, your indicator has zero value.
If you asked a hard question, it might take time for users to come up and/or write down a solution. Again, having more users online doesn't help with this either.
Honestly, getting an answer to your question isn't a race against the clock and shouldn't be optimized for getting an answer as fast as possible.
Last of all, we're not a forum, but a Q&A site. Our answers aren't meant to only help the OP, they are meant to help hundreds or thousands of other users. To them, it doesn't matter how fast the question was answered, as long as it solves their problem.
This is a function of chat -- in the header it will tell you who how many users are online, like so
and clicking that will take you to a list of users that were seen recently (well, that is the default users tab anyway)
There are of course dedicated chat rooms for Super User