If he was in your party and left because you didn't rescue Dynaheir in time, he's gone for good. If you dropped him from your party I think he will go back to where you originally recruited him (or possibly the FAI). If you mean that you haven't recruited him yet - go to Nashkel.
Only the EE companions go anywhere, all the vanilla ones just stand wherever you left them. So if you left them at the end of the Nashkel Mines, you have to trudge all the way through it again to go grab them.
Poor Xan.
I know there's a console command to spawn NPCs, but it eludes me at the moment. They won't have any gear you gave them if you spawn them this way, though if you're going to use this method, you can always spawn the items too.
Yeah, but that'll create a new version of Minsc. If the Minsc he lost has some good gear on him, the Minsc created by that command won't have that equipment. Though you could create a whole party of Minsces that way. Butt kicking for goodness times five.
Maybe your Minsc got chunked? It happened to me once that an NPC I believed to be 'merely' killed was permanently removed that way and I only realised it after the fight was over.
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Poor Xan.
I know there's a console command to spawn NPCs, but it eludes me at the moment. They won't have any gear you gave them if you spawn them this way, though if you're going to use this method, you can always spawn the items too.
Anyways...
"Wherever evil treads, Minsc treads louder!"