I don't know how to help, other than start at the beginning.
They have to be in your group/raid. One person selects the person that is to be summoned. The other person clicks the purple circle glowy thing. It takes like 5-8 secs and you won't know if it worked. Just like 10-12 secs later, the person shows up. I am not sure of all the limitations, but one big one is can't be in combat. If you want to practice to see what is going on, you can summon yourself (or your alt). Hope that helps and good luck.