[Bungleau]

Anyone can become an assassin. In my first game, that's how my wizard finally achieved another class. I picked his stats so poorly that he would have been level 40 before he could have become anything else.

All you have to do is walk into the thieves' guild with the dagger in hand, and you're invited. And if you're playing unpatched, you can get sneaky and hand off the dagger to someone else before the guildmaster finishes talking, and not only are you an assassin, but you've still got the dagger. So someone else can become an assassin, and so on, and so on....

Same with the other special roles. There are no class restrictions for who is eligible.