Yes, you can book on silkair.com and pay for your friends, family, or colleagues if they are travelling with you in the same travelling party.
You need to indicate on the payment page that you are part of the travelling party.
As a paying party, you may be required to bring the credit/debit card used for the booking at Airport at the time of check-in. You will be duly notified for the card verification requirement during your on-line purchase if there is a need to do so.