Clicking on "Users" from the top menu will list the users you are allowed to administer (if you have no admin rights, you will see your own user details screen), as seen below:

While viewing the user list you can see which users are online, they will be indicated by a green icon. If you want to see only the online users click on the "Online users" link at the left top corner.

You can search the user list by using the search bar at the top of the screen. Opinio will display all users whose login or full name contains the search string. You can only search through the users you can administer.