you should have been asked during install if you wanted it for yourself only or any user, if you clicked any user then it should have placed an icon on the desktop of eack user, if not then you just need to navigate to the opera folder and create a shortcut for each user (log onto each user and do the same thing).
if you have set the users as limited then you may have to change some settings to allow changes in the other accounts (ie. changing the accounts to administrator, doing the changes then changing them back to limited.
hope that makes sense.
it might be though that opera is available to everyone if you set it to be the default browser. im guessing there.....