To register users with Openfire you will need to use a plugin called ‘User Service’.
Start Openfire and log into the administrative console. There, click on plugins. If you see nothing, click the link there to refresh the plugin list.
In the list that appears, you will see the ‘User Service’ plugin - click its installation link to get it working.
It comes with simple documentation that will do the job, but essentially the way it works is by offering you a special URL you can use to add, remove and update Openfire users.