All I know is that "to summon" is "shookan suru".
It's a very difficult question even to Japanese.
Though there does exist the word, 召喚者（shoukan sha), we rarely use it.
者(sha) here means people.
I guess 「召喚された人」(shoukansaretahito) is much easier to understand, meaning 'people who were summoned'.
I agree with the Japanese poster above. It's rarely used out there.
However it's very often used for the games like in "Final Fantasy". You might not know the game but if you meant to ask it the answer is å¬åå£«(syoukanshi).
If you don't have any interest in the game, you can through away this answer. haha
You could take the easy way out and add "Hito" to the end.