If you require your user's to register with the site (through Joomla's registration) prior to purchasing a Membership, then you can modify the profile plugin that ships with Joomla. The profile plugin can be used to request additional information during the registration process. You can find the profile plugin at Administrator>>Extensions>>Plugin Manager>>User-Profile.
More information on modifying this plugin, or building your own can be found at the link below.http://docs.joomla.org/Creating_a_profile_plugin
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