So how exactly do you summon/conjure a creature as a conjurer? I'm all the way to the Hamlet of Brin and I can still only summon bombers or charm creatures that are already there, unless I'm missing something.

You've probably already gotten there, but the Summon Creatures ability is received when you start the Temple of Ix chapter. It gives you the Summon Creature ability and then you can start summoning anything you have a scroll for. Standard charming rules and functions go as normal from there.