Joomla support in CiviCRM has been lagging behind but this plugin fills a gap in quite a neat way.
Only drawback is that it relies on you using the CiviMember component of CiviCRM and there are a few issues with this.
Suggest a simple modification to alternatively authenticate against membership of a specified (as a parameter) group (or groups). This would also allow you to define arbitrary subsets of members who had login rights.