config.rb

helpers do
  def is_current_page(page)
    current_page.url == page ? {:class => 'is-active'} : {}
  end

  def groups
    sitemap.resources.select{ |resource| resource.data.type == 'page' }
      .group_by { |g| g.data.group }
  end
end

example.md

title	type	priority	group
Courses	page	1	Group A

navigation.haml

%ul
  %li{is_current_page('/')}= link_to 'Home', 'index.html'
  - groups().each do |name, pages|
    %h1= name
    %ul
      - pages.sort_by{ |r| r.data.priority.to_i }.each do |page|
        %li{is_current_page(page.url)}= link_to page.data.title, page.path