attributes.rb

default[::groups] = [ "foo","bar","baz" ]

recipe.rb

template "app.conf" do
  source "app.conf.erb"
  owner "root"
  group "root"
  mode 0640
  :groups => node[:groups]
end

template.erb

  groups = [
  <% @groups.each do |group| -%>
    <%= group %>
  <% end -%> ]