Alright, so my sister wants to buy this game and she wants to let one of her friends play it as well as her.

Can you create 2 accounts off of one Cd key? Can you create 2 characters off one cd key?

You can only create one account per CD key, then it becomes useless. But each account gives you four characters, which is enough to last.

I'm guessing only one computer can be logined into an account at once?

Yes. If you wanna play with your friend, you both need a copy.

My guild leader plays with his wife and they both have their own accounts.

Aight, gotcha.

ok 1 thing I dont understand,

I got a cd key I made an account and I got my 4 characters.
Is it possible to make another account so I can have more charcters on the same account?

No. Not unless you get another CD key. If you do get a new CD Key, you can use it to create a new account with four characters, but I don't think you can combine it with your original.