What is the ruling of the Sharia on subscribing to an internet network that may be used by others to watch songs and pornographic films, and does the subscriber bear their sin, and what if he only shares in the payment without the network being registered in his name?

If your friends' accessing forbidden websites is merely a possibility, this should not prevent you from installing internet in your name or sharing it with them. The default is to assume the best of a Muslim, and if someone dares to view a forbidden act without your knowledge, the sin is on them. However, if you are certain or highly assume that they will use the internet for disobedience, then it is not permissible to share in its installation, due to His Almighty's saying: "And cooperate in righteousness and piety, but do not cooperate in sin and aggression."