redmine/app/views/users/_groups.rhtml

<% form_for(:user, :url => { :action => 'update' }, :html => {:method => :put}) do %>
<div class="box">
<% Group.all.sort.each do |group| %>
<label><%= check_box_tag 'user[group_ids][]', group.id, @user.groups.include?(group) %> <%=h group %></label><br />
<% end %>
<%= hidden_field_tag 'user[group_ids][]', '' %>
</div>
<%= submit_tag l(:button_save) %>
<% end %>