Please describe specifically what you are seeing in Labs that you find missing in Alfresco 4. Then, maybe we can help determine whether or not that feature is still present.
As in the wikki link http://wiki.alfresco.com/wiki/Alfresco_ … 0_Features under the Site Management its written there. " >Site Management * Site Members - Invite existing users or new (non-registered) users to join site; Automatic registration for new users; Manage site invites pending, accepted, and cancel invites "
In that case u can create one group invite that group to the site. Then what ever the user you wanted to invite you can just add to that group he will be invited automatically.