How do I pre-populate the login/sign up form? I'm working on a project that let any user (anonymous) register for an event. An event is a content type that has duration, name and price and possibly a date. So, when a user browse through different events, they can register for an event by simply clicking on the "Register for this Event" What needs to happen when they click on Register.. is: the info related to that event needs to pre populate to the member login form so that they don't have to fill the event info again. 

The user must be a member before they can register for the event but I am trying to give a nice user experience to the user so that he/she can simply click on the Register link and then they will be redirected to the user login/sign up form the event info pre-loaded and they provide the rest of the info.
I have already extended the standard Drupal login form to my need and also created the event content type. Thinking to use flag module to make Register link but have NO idea how to pre populate that event info into the login form. Any help or guide will be highly appreciated. Thanks in advance.
I have already extended the standard Drupal login form


Sounds like you're already using hook_form_alter(), so all you have to do is add the #default_value attribute to your form array. 

Ex:

$form['my_field_name'] = array(
    '#type' =&gt; 'textfield',
    '#title' =&gt; t('something'),
    '#default_value' =&gt; t('Hello World'),
);




OR Use the Profile2 module suggested by @Kevin


  Profile2 allows for creating multiple profile types, which may be
  assigned to roles via permissions (e.g. a general profile + a customer
  profile)