Are you running a WordPress network? You no longer need to manually add new users to each of your sites.
With this plugin, users are assigned a default role on a new site when they log in for the first time (they authenticate against the network username and password). You set the default role for each site and this plugin applies it. If the user already has a higher role assigned they keep their role on logon
You can assign different roles for each site or keep a site private by assigning no role.