app/views/user_sessions/new.html.erb
<% content_for :left do %>
<div class="box">
<h2>Don't be shy, come on in!</h2>
<div id="login">
<%= render partial: 'login' %>
</div>
</div>
<% end %>
<% content_for :right do %>
<%= render partial: 'users/join' unless params[:already_joined] %>
<% end %>