// app.yml
all:
  sf_guard_plugin:
    profile_class:      sfGuardUserProfile
    profile_field_name: user_id


// schema.yml

sfGuardUserProfile:
  tableName; sf_guard_user_profile
  columns:
    user_id:
      type: integer(4)
      notNull: true
    email:
      type: string(255)
    birthday:
       type: date
  relations:
    User:
      class: sfGuardUser
      local: user_id
      foreign: id
      foreignAlias: Profile
      onDelete: CASCADE

// sfGuardUserProfile.class.php

public function preInsert($event)
{
  $this->email = $this->User->username;
}

// or sfGuardUser.class.php

public function preInsert($event)
{
  $this->username = $this->Profile->email;
}