app/views/users/_new_invite.html.erb
<fieldset>
<legend style="font-size: 1.3em;">Enter the email addresses of the friends you'd like to invite:</legend>
<%= form_tag invite_friends_path, class:"form-horizontal" do %>
<%= text_area_tag 'emails[]', nil, :required => true, placeholder: "jane@example.com, nancy@example.com, barbara@example.com …".html_safe, class:"form-control" %><br>
<%= submit_tag "Invite Friends!", class:"btn btn-info" %>
<% end %>
</fieldset>