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However, it would seem that you could engineer your rewrite such that it didn't need to be modified when you wanted to add someone.
There have been a few rewrite related posts that involved detecting if the request points to an existing file, directory, or symbolic link. Failing those tests, you can then direct it to pass that request to some handling script. This script could look for a context where that request makes sense, for instance searching the username list or performing a site-wide search or some other behavior.
Well, first make sure you have the right access to the file, CHMOD sounds like it?
Imho, you shouldn't be able to access/edit .htaccess file from php, because, if anyone could edit any .htaccess located on the web server root, then they will be able to gain access anywhere.