Follow one of this links to view a list of players "online" on Dragon:

• My opponents being online (during the last 10 minutes)
  
• Players online (during the last 5 minutes)
  

The number of minutes to search for can be adjusted in the "Last access"-filter.
 
Though the above links provide what is asked for, nobody is "online" on the Dragon Go Server. Dragon only receives page requests and delivers them back to the requesting user. There is no active connection between a player and the server. What comes close to it being "online" is the last access time of a player.
 
Other places, where the online-status and/or last-access is shown on Dragon:

• column "(User online)" on the status page shows the -icon if a user was "online" within the last 10 minutes. The tooltip of the icon shows the time when the opponent was last accessing the site (when hovering over the icon), e.g. "<3 mins".
   
  
• Last access on the User info page of a certain user
  
• check column Opponent's last access on running games table of a certain user: Status page -> Running games -> (column may need to be added and sorted).