Where do I find magnus after I told him to leave the group??

So, you've asked Magnus to leave the group, and want him back again?

Have you tried looking for him at the spot at which you left him? If he's not there, he may have returned to the gate of P. Schuylers & Sons again (the place were you found him).

Other than that, I don't know - though I guess he could be in Caladon.

Oh, and by the way - try to be a little more specific when you ask for help. It's a bit hard to help someone out when you don't know whatever he's tried this and that etc etc.