Is bringing a film that contains indecent scenes by mistake, and telling friends to avert their gaze from them, a cause for sin? And does the sin extend to me if others watch it afterwards? And is repentance accepted for this sin?

You made a mistake by watching and obtaining a film containing objectionable material and showing it to others. What is incumbent upon you is to seek forgiveness and repent to Allah Almighty by feeling remorse and resolving never to return to such a thing in the future. If you repent, Allah will accept your repentance. The Sharia indicates that whoever repents sincerely, their sin is forgiven, no matter how great it was. Allah Almighty says: "Say, 'O My servants who have transgressed against themselves [by sinning], do not despair of the mercy of Allah. Indeed, Allah forgives all sins. Indeed, it is He who is the Forgiving, the Merciful.'"

If you repent, then – Allah Almighty willing – your friends' insistence on watching the film will not harm you, as long as you are unable to take it away from them. Scholars have stated that whoever repents from disseminating something forbidden, their repentance is accepted, and the lingering effect of their sin that they are unable to remove does not harm them.