aardbei/app/views/members/_form.html.erb

<%= form_for([@group, member]) do |f| %>
  <% if member.errors.any? %>
    <div id="error_explanation">
      <h2><%= pluralize(member.errors.count, "error") %> prohibited this member from being saved:</h2>

      <ul>
      <% member.errors.full_messages.each do |message| %>
        <li><%= message %></li>
      <% end %>
      </ul>
    </div>
  <% end %>

  <div class="actions">
    <%= f.collection_select(:person_id, @possible_members, :id, :full_name, prompt: true) %>
    <%= f.check_box :is_leader %> <%= t 'groups.leader' %>
    <%= f.submit %>
  </div>
<% end %>