Asking for a family member who will be going on a tour next year. He wants to book his own flights and arrive a day early, so they won't give him an airport transfer. He wants to know how to get from KEF to the start of the tour in Selfoss.
Bus to Reykjavík and change there to bus 51 or 52 to Selfoss.
Selfoss is not easy to get to. Strange that the tour is starting there.
Website for public bus in English is www.straeto.is/en. Your family member could search options from the airport to Selfoss.
Your family member could also look into the feasibility of renting a car at the airport and returning it in Selfoss. I highly doubt it, but he/she needs to do the research.
I don't know how you define easy to get to, but there are buses roughly once an hour. So not that hard to get to in my opinion.