When reading a message with many recipients, you can save all of them to your address book — and optionally add them to a group — in one step.
If you choose Create new group and type a name that already exists, QuickMail will not create a second group with that name. It tells you the group already exists and keeps the window open so you can enter a different name, or pick the existing group from the list instead.
Tab moves through the address list (one Tab stop for the whole list — arrow keys move between individual checkboxes), then to Add to group, then to the group combo, then to the name field, then to Save and Cancel.