Depends on what you mean by who.
Who being computers or who being users.
In Airport Utility, Summary, you can click on the "Wireless Clients" label and it will display the connected Wireless clients by MAC address.
In Terminal, type the command, who to show which usernames are logged into your computer.
Edit: I forgot about File Sharing Users. I'll have to do some research to figure that one out.
Message was edited by: Barney-15E