/**
 * An apex page controller that supports self registration of users in communities that allow self registration
 */
public with sharing class CommunitiesSelfRegController {

    public String firstName {get; set;}
    public String lastName {get; set;}
    public String email {get; set;}
    public String password {get; set {password = value == null ? value : value.trim(); } }
    public String confirmPassword {get; set { confirmPassword = value == null ? value : value.trim(); } }
    public String communityNickname {get; set { communityNickname = value == null ? value : value.trim(); } }
    
    public CommunitiesSelfRegController() {}
    
    private boolean isValidPassword() {
        return password == confirmPassword;
    }

    public PageReference registerUser() {
    
           // it's okay if password is null - we'll send the user a random password in that case
        if (!isValidPassword()) {
            ApexPages.Message msg = new ApexPages.Message(ApexPages.Severity.ERROR, Label.site.passwords_dont_match);
            ApexPages.addMessage(msg);
            return null;
        }    

        String profileId = null; // To be filled in by customer.
        String roleEnum = null; // To be filled in by customer.
       <strong> String accountId = ''; // To be filled in by customer.</strong>
        
        String userName = email;

        User u = new User();
        u.Username = userName;
        u.Email = email;
        u.FirstName = firstName;
        u.LastName = lastName;
        u.CommunityNickname = communityNickname;
    u.ProfileId = profileId;
        
        String userId = Site.createPortalUser(u, accountId, password);
      
        if (userId != null) { 
            if (password != null && password.length() > 1) {
                return Site.login(userName, password, null);
            }
            else {
                PageReference page = System.Page.CommunitiesSelfRegConfirm;
                page.setRedirect(true);
                return page;
            }
        }
        return null;
    }
}

During the sites self-registration process, I have only come across the ability to create a portal user that creates both a contact and a user synchronously.  I would like to be able to create a portal user using an existing contact. 

Overall, need to create new user with existing contacts, i.e email id....Have to modity the default self registration controller.

Can any experts post the code in which you were able to enable new users under existing contact. It will help me alot...