well they are npc's an you have to be friends with them first... An then you ask them to teach you to be a witch...An then you have to buy the book
from them an all the other stuff too...

indeed indeed
But the witches that aren't the HighWitches can also witch people up
After a certain point of magical ability, a witch has a new spell that enables her to turn another sim into a witch/warlock, but said sim can't ask them to be turned into a witch
And if you materielize to much, your sim ends up falling out of the sky next time they try to do it:)
The best spell i'd say is the one to freeze time, epsically on comminuty lots, its really cool. But if your witch tries to summon a sim on a frozen time lot, their wands go wonky and they can't do it
EAxis did add some fun features.