invite = Invitation.find_by_code(params[:id])
    @user   = User.new(:email => invite.email)
    @user.invitation_code = invite.code

CALLS show.rhtml

<script type="text/javascript" charset="utf-8">
  function pageCallback() {
    UtilityPane.show(Templates.Users.signupNewUser(<%= @user.to_json %>))
  }
</script>
<%= @user.to_json %>

RESULTING json:

 {"openid_url": null, "nickname": null, "invites": null, "password": null, "email": "uiuiio@iiiio.cc"}