How do I add a columt to Who's Online?
 

The current displayed columns are
User Name Location IP Address Instant Messaging

I would like to add another column which would display the Date of Registration and clicking it would sort the list of members by their joining date. I am orry if this post is in the wrong place I could not find a better place for it.

You can do that in the Member List. (vboptions > user listing > member list fields > join date)