Hi!
I create a section to register user. Who is register can post comment about a particular published book.
I do my form to register member, I tahe form value and I ceck if in my db user with the same name and password exixts. Now how can do to allow user to post?An remeber that the user is registre?I try look on forum module but something is not clear for me and I can't please help me..

If you mean login after registering try this:

$member_login = new MemberLoginForm();

if($member_login->performLogin($data))
{
// Redirect to the post page
Director::redirect('post-page/');
}

Thanks!
but I have an other problem. When I try to log a user seems forgot session. so I use:
$_SESSION["utente"] = $data['nome'];
$_SESSION["id"] = session_id();
in login function to remember user session. and I pass this varable in template but something is wrong..