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When Does CiviCRM Create a Contact Record for a User? When is a User Linked to an Existing Contact?http://wiki.civicrm.org/confluence/pages/viewpage.action?pageId=14452376
note that a civicrm profile (and hence a contribution/membership/event registration) page has the ability to create a joomla user (check create CMS account in advanced settings for a profile)lobo
There are some ways to achieve what you need using some other joomla extensions. What I typically do is import my contact records into Civi, then export and create usernames and initial passwords for all users (who should receive access to J). I then use a J extension to import into the Joomla user manager table. Optionally I may import into Civi for initial reference and to send out a broadcast email letting people know what their assigned user/pwd is.I can spell out the specifics in more detail if you need. I've done this with several websites where we wanted to get the user base up and running right away.Also note that there is a CiviUser extension for Joomla that replaces the core com_user component: http://wiki.civicrm.org/confluence/display/CRMDOC/Joomla+User+Registration+RestrictionThis allows you to expose a website registration page, but limit registrations to only members in good standing (not sure if you're coming from a member-based organization perspective).
If a user logs into Joomla, and no uf_match record already exists (so the J! user is not connected with a Civi contact record), Civi seeks to find a match (based on the email address) when the logged in user access. That mechanism is good, but not perfect. And for a database of 2000 records, I would not rely on that as my primary "connecting" mechanism.