Yes. You just book online without logging in and enter their name and your credit card number.

Then you need to call the hotel and ask them to fax you a Third Party Billing Authorization Form. You complete this form and fax it back along with the front and back of your credit card and the front and back of a drivers license or passport info page.

Now, if you are asking if you can book a room for another using your SPG number? That's another question, and is against the rules if you aren't staying with them. But I've read some have managed to pull it off.