htaccess is definately NOT the place to store userdata.
This file is only for setting access rights on webpages.
Instead, setup some kind of database, like MySQL.
The manual has some examples of how to store data in a database, that should give you an idea of how it works.
If you can't setup a database, you can store the user data in a textfile.
PHP's filesystem functions let you read a file into an array that you can work with
to find/change users.