What do you want to do with your members once you have them registering?
To do anything "useful" (run reports, use tools to organize members into groups for communications or listing rosters, event signups, member profile pages, etc..), you'll want to strap a CRM on the back end of the site. I'd suggest looking at CiviCRM (http://civicrm.org). It has flavours to work with Drupal, Joomla and Wordpress. I'm very familiar with Drupal integration, specifically for member driven organizations so I can't really speak for how well it integrates with Wordpress but the Drupal version is pretty amazing.