You can do a lot with the INI files for storage. It'd be better to use hash files, to be honest, but depending on how well you can handle them, it might not be your cup of tea. Or you might try using arrayed variables, then access the users access that way.
It really wouldn't make sense to have each level on a seperate line, because if you have, say, 10 people with 3 levels each, that's 30 lines. That's small scale. If you have, say, 30 people with an average of 4 levels each, then you have over 100 lines for only 30 people. And if you don't keep it updated, then you will either run out of space for your users (I think there is a limit), or if there is no limit, your user list will just grow beyond belief. Either way, it will happen quickly.
