It's joe-say.
I dont think there's a difference between jo and jyo though; it's pronounced the same way basically and often written one way or another. Basically, the only jo that existed in the hiragana s the shi, with that weird sign on top and a yo, can be written in latin as jyo or jo... right?
If you really wanna hear the voice actors say it then check out hang out crisis audios on youtube ;)
[BLCD/18+] Hang Out Crisis Track 01 - YouTube
https://www.youtube.com/watch?v=IJ7llN7QQpI
I'd say "Joh-zee" but listen for yourself (●'◡'●)ノ

How do you pronounce Jyousei? Is it like the English name, Josei, - as in: Joe (rhymes with Low), and Sei (rhymes with Day) - pronounced Joe-Say?
Or maybe pronounced Joe-zee?
Does anyone have a odefinite pronounciation of that name?
(Jyousei tongue-twisterish and reminds me of jallopy (old car) - don't like, but if that's the right way, I'll put up with it. lol
I adore this manga by the way