Well, you could code it, but it's kind of a cheesy solution. Assuming you have a group called "My Group" you want everyone to join, you could write some code and embed it on a page that everyone hits. As soon as they land on it, they get added to the group automatically. PHP/Drupal code might look something like this (Drupal 7 example):
//get the user's id
//get the group's GroupID
$result=db_query("SELECT gid FROM og WHERE `label`=:groupName",array(':groupName'=>'My Group'));
//check and see if the user is a member of the group already
$result=db_query("SELECT COUNT(id) AS og_count FROM og_membership WHERE etid=:uid AND gid=:gid",array(':uid'=>$uid,':gid'=>$gid));
if($og_count==0 && $uid!=0)
This is not the preferred way to do it, but in a pinch you could add this code to a heavy traffic page and pretty much ensure your users get added. The down side is that you are doing an extra database dip or two to ensure your users are added to a specific group. Not very efficient, but it would likely do what you want.
As a caveat, this is based on what I know from playing around with Organic Groups. This code is from the top of my head, so there may be a parsing error here or there - be sure to check it out first. Also, this is direct dipping into og's table structure, which is frowned upon, as og could change their table structure in the future, during a code update...