app/views/members/_form.html.erb
<div class='row'>
<%= form_for :member, url: organizer_members_path, method: :post, html: { class: "dbl-col" } do |f| %>
<fieldset>
<div class="twin-container">
<div class="twin-field">
<%= f.text_field :email, placeholder: 'Email address' %>
</div>
<div class="twin-field">
<%= f.submit 'Invite to team', class: 'btn btn-primary' %>
</div>
</div>
</fieldset>
<% end %>
</div>