You can't do it using the Contacts Sidebar within the Write/compose window, but you can do it from the Address Book window. Open the Address Book window and select your list so that you can see its contents, select the addresses you want then simply drag and drop them into the desired recipients field in the Write window. This works great if you have the Address Book and Write windows open side by side.

The Expand mailing lists add-on adds a button to the Write window to show all contacts in a list that has been added to the addressing box. Middle-click a contact to remove it from the box.

Prior to TB 78, it was possible to expand a list in the Contacts sidebar by simply pressing Ctrl+I (Edit), OK.

The Add-On from the link doesn't work with TB 78. I used to use the Ctrl+I, OK solution before TB 78. Any other ideas? Thanks

The add-on works here in 78. You have to add the list to the addressing box and then click the expand icon. Check that the add-on is installed and enabled.

sfhowes nailed it! Thanks! Works fine!!