I don't really see why it needs to be made easier. You have access to it from the beginning of the game, and while pickpocketing may be harder, it's not like to need to actively try to become a member immediately - you could simply make it a point to use the skill whenever you have an opportunity to do so.
But I also think a Thief should have automatic entry into the guild. It makes thematic sense. Perhaps thieves could even be offered some item by the head of the thieves' guild in exchange for undertaking a pickpocketing quest.
For Assassins, I think being a member of the thieves' guild doesn't make a lot of sense. But I could see assassins having some sort of special quest from Hotzenplotz, or even from the head of the thieves' guild, in which they are to use their assassin skills in some way.
Actually, it would be cool if each class got a special quest with a reward that goes well with that class, that gives it a tool that helps it with what it does best. Wizards could get a Ring of Mental Stability, for instance.