The 18 hole golf courses are usually busy, particular the golf courses around the Greater Reykjavík area. Golfers are advised to book tee time in advance especially if they are travelling together in a group. Most 18 hole golf courses accept advance booking but you might have to confirm the booking with deposit i.e. through credit card.
You have to book tee time with each individual golf course. You can either call them by phone or send them e-mail. Information about telephone number and e-mail address for each golf course is on their information pages on this website.
Some travel agencies might be able to provide joint booking.
At most of the 9 hole golf courses the tee time is available up on arrival except for groups when advance booking might be necessary.