Who are the people that know how to use Kaio-Ken? Are Kakaroto (Goku) and Kaiō-sama (King Kai) the only people that know how to use the Kaio-Ken? 

Did Kaiō-sama teach his technique for others beside Goku? For example, while training Krillin and the others.
King Kai himself was not able to use that technique, and he said that Goku was the first one who ever mastered that technique. However, King Kai tries to teach the technique to everyone who visits his planet.