They are appointed, one from each country. They are in effect the whole year long. It's quite a big body, which is like another court.
It's something that came out of negotiations with the EFTA countries. When there were more EFTA countries at that time, it meant they established the EEA agreement. At this time, there are those EFTA countries left. But there are others, like Sweden and Finland, that were also members of EEA--but are now members of EU--when that was established.