I've not used qmail, but I'll tell you how to do it in thunderbird. The procedure must be very similar in qmail.
First click on your address book. You'll have an option to create new contacts. Near that, you'll have an option to create a new list. Click on it an name it something like "friends". Now, click on the mail-id's from your address book, and drag and drop it into the list you've created. This process works in Mozilla Thunderbird, but I don't know about qmail, but I think it must be similar.
I am away from my box, so I'll check it tonight and post back about whether this is the same procedure in qmail or not