Hi,
If you are creating the customer portal user through internally then you can below sample code to create a customer portal user.
--------------------- Customer Portal User created internally-----------------

Profile pf = [Select id,name from Profile where name ='Volunteer' limit 1];

account acc = [select name from accout where name="individual"];

Contact con = [select id,name,email,lastname,firstname,accoundid from contact where accountid=:accid limit 1];

User u2 = new User(contactId=con.Id, username=con.Email, firstname=con.FirstName,
lastname=con.LastName, email=con.Email,communityNickname = con.LastName + '_' + Rnd,
alias = string.valueof(con.FirstName.substring(0,1) + con.LastName.substring(0,1)), profileid = pf.Id, emailencodingkey='UTF-8',
languagelocalekey='en_US', localesidkey='en_US', timezonesidkey='America/Los_Angeles');
Database.DMLOptions dlo = new Database.DMLOptions();
dlo.EmailHeader.triggerUserEmail= true;
Database.saveresult sr = Database.insert(u2,dlo);
--------------------- Customer Portal User through sites-----------------
Public string Email{get;set;}
Public string FirstName{get;set;}
Public string LastName{get;set;}
Public string CommunityNickname{get;set;}
Public string Passwd{get;set;}
account acc = [select name from accout where name="individual"];
User u = new User();
u.Username = Email;
u.FirstName = FirstName;
u.LastName = LastName;
u.Email = Email;
// u.IsPortalSelfRegistered = true;
u.CommunityNickname = communityNickname;
String pwdd = Passwd;
system.debug('<< before create user >>');
String userId = Site.createPortalUser(u, acc.id, pwdd); // This method create a customer portal user

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