Tink,
Everyone in your party (including DD) will need their own skymiles acct, and the miles will be credited to each of their accts. Accounts are free, and you can sign up easily at the Delta.com website. Get the accounts before booking the flights so you can enter their skymiles number when you purchase. Find out if your co-worker already has a skymiles number, so the miles will be credited to his existing account.
As far as the actual purchase, it can all be billed out to one credit card, but since there are two different travel dates involved there will be two billings.
For detailed answers you can contact skymiles via e-mail or phone once you have the skymiles numbers.
I just remembered, actually each ticket shows up as a separate credit card transaction.