What must a girl who has committed adultery with two young men do to repent, and should she marry one of them as expiation for her sin?

The duty is to repent to Allah with sincere repentance by abandoning the sin, resolving not to return to it, feeling remorse for it, adhering steadfastly to Allah's law, and increasing good deeds that erase sins. It is not a condition for repentance to marry the one with whom you committed adultery, but it is permissible for you to marry him if he proposes to you.