I believe that as long as you have not set a limit, the disk space is actually increasing for these users automatically, it's just not updating the text in your panel.
If you set a limit, that's not going to increase on it's own, you'll have to decided how to increase it your self. But as long as it's the 'overall plan limit' you should not run into a problem.
Basically, I believe that you don't need to worry about it. The limit is the one that's automatically increasing, not what it says.