The easiest way is to use the Community Builder extension. This is one of the most popular Joomla! extensions, and it has many features. Allowing the site administrator to define additional fields for the user profile is only the beginning.
CB, as it is called, has a plugin architecture. Both CB developers and others have developed plugins that allow you to extend what CB can do for you. Connections with Facebook and Twitter, paid subscriptions, rating of other users - there are many possibilities.
CB is free software under the GPL. However, you pay for access to documentation and support. In addition, some of the plugins are proprietary software.
Update . The current version of CB is 1.7.1, and it supports Joomla! 1.0 to 1.7, but not Joomla 2.5.x. However, as of February 7, 2012, Community Builder 1.8 is in RC . It supports and has been tested with all Joomla! version through 2.5.1. RC stands for “Release Candidate”, so it’s not yet ready for use on production sites, although the developers say that it uses its own joomlapolis.com website, “The goal is to release a stable version of CB 1.8 before the end date of Joomla 1.7” - say the developers.
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