redmine/app/views/groups/index.api.rsb

api.array :groups do
  @groups.each do |group|
    api.group do
      api.id         group.id
      api.name       group.lastname
      api.builtin    group.builtin_type if group.builtin_type

      render_api_custom_values group.visible_custom_field_values, api
    end
  end
end