You can create your own page where you can get all required details and then call the webscript of create user.It will create user in alfresco and allow access to it.
What I need is to let the user sign up by himself to the system , he should receive an email into his email , like all normal sign up in any website , I don't need to create webscript , I just want to change login forum to other that had a sign up in the same forum . you may look at that link to see what do I mean ?