You can enable the “Create your account” button in portal customization parameters.
Then, go in Portal > Advanced parameters > Register new account:
-  Module: Choose the backend to use to create the new account. 
-  Page URL- :  URL-  of register page 
 
-  Validity time of a register request: duration in seconds of a new account request. The request will be deleted after this time if user do not click on the link. 
-  Subject for confirmation mail: Subject of the mail containing the confirmation link 
-  Subject for done mail: Subject of the mail giving login and password