I'd open the Contacts Sidebar, select the Collected Addresses address book, then select those Contacts using the usual multiple selection techniques (ctrl or shift clicks) and add them to the message using the "Add to…" buttons. Do think about if they should be To or Bcc. If you're going to correspond with this group regularly I'd consider setting up their own address book, or making them into a Mailing List. I do wonder why people actively use Collected Addresses. I hoover up all the useful addresses and put them into an appropriate address book, then nuke the crufty stuff that has crept in. I try to keep CA empty.