Coelocanth: You're making the game a helluva lot harder by not taking a mage with you.
Elmofongo: Mostly because of Jaheira, I would gladly have a mage if I can get rid of Khalid, but than I have to get rid of Jaheira, Dynaheir is an Invoker which I don't know what can they do and I have to take Minsc along. Xan is too weak.
I am thinking of getting this guy name Quayle is an Illusionist, what can he do?
Dynaheir is a mage. An Invoker is a specialist mage, just like Illusionist is. Invokers cannot use spells from the Conjuration or Enchantment school. Illusionists cannot use spells from the Necromancy school. Xan is an Enchanter, and as such cannot use spells from the Evocation (Dynaheir's) school.
Quayle is a Cleric/Illusionist, and he is virtually useless: his WIS is only 10, so his cleric spell list will be lacking. The rest of his stats (except for 17 INT) are abysmal, too.