app/views/groups/users/_new_user_assignment_form.html.erb
<h1>Assign User</h1>
<%= form_for(@user_assignment, :url=>group_users_path(@group), :remote => true) do |f| %>
<%= f.error_messages %>
<%= f.collection_select(:user_id,@users, :id,:to_s) %>
<%=f.check_box :is_principal %><%= f.label :is_principal%>
<p>
<%= f.submit "Assign User" %>
</p>
<% end %>