We can find the answer to this question in Chapter Two of this excellent coming of age novel. Having set up the gang, Tom says that you can only enter it if you sign an oath saying that the families of the boys in the gang who told secrets would be killed in revenge for the revealing of those secrets. This of course presents a problem for Huck, as he has no family. It looks as if Huck might be not allowed to join the gang because of this, but fortunately, Huck is able to come up with a suitable alternative:
Well, nobody could think of anything to do--everybody was stumped, and set still. I was most ready to cry; but all at once I thought of a way, and so I offered them Miss Watson--they could kill her.
With this solution, Huck is able to join the gang as he has managed to think of somebody who could act as a substitute family and could be killed in turn. The oaths of course highglightTom Sawyer's remarkable imagination and the way that he is obsessed by adventure books, which foreshadows his unnecessary complications of what could be a simple escape plan at the end of the novel.