app/views/challenges/form/_invitation_fields.html.erb
<div class="nested-fields">
<fieldset>
<div class="twin-container">
<div class="twin-field">
<%= f.text_field :email, placeholder: 'Enter email address' %>
</div>
<div class="twin-field">
<label></label>
<%= link_to_remove_association "remove", f %>
</div>
</div>
</div>