On the right of your board, below the move list, you find a list of 'followers'.
It says 'followers' but it is a mix of observers and followers.
Or use the command 'allobservers' combined with username or boardnumber.
Example type on the command line of your board or console: allobservers Eendje
You can also use the shortcut 'allob'. Example: allob 242
For more information read "help allobservers".