How do you add more than one character to a quest? Every time I create a new character, it will only start a new quest with just that character. Also, can you add a character to a quest you have already started? Any help would be greatly appreciated.

You have to drag the characters to those empty boxes. There's two of em, so that shouldn't be too hard.

Only two characters per game are allowed. Once you get to a certain level though, you will probably only want to play solo for the exp.