If one invite a friend for dinner and the friend without informing you bring another person for dinner as well. What Islam says about the accompanying person.

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and messenger.

When a person is invited over for a meal, then good manners should prevent him from bringing someone else who is not invited by the host (with him) unless the host approves the invitation. Abu Mas‘ood Al-Ansaari reported, “There was a man called Abu Shu‘ayb, and he had a slave who was a butcher. He said (to his slave), "Prepare a meal to which I may invite the Messenger of Allaah, sallallaahu ‘alayhi wa sallam, along with four other men.” So, he invited the Messenger of Allaah, sallallaahu ‘alayhi wa sallam, along with four other men. However, another man followed them, whereupon the Prophet, sallallaahu ‘alayhi wa sallam, said, “You have invited me as one of five guests, but now another man has followed us. If you wish you can admit him, and if you wish you can refuse.” On that the host said, “But I admit him (i.e. he, too, is welcomed to the meal).” [Al-Bukhari]

Ibn Battaal wrote, “It is inferred from this Hadeeth that it is improper for a guest who is invited over for a meal to bring another uninvited guest with him.” [Sharh Al-Bukhari]

However, if it is known that the host would approve having extra guests, there is no harm in doing that. The Prophet, sallallaahu ‘alayhi wa sallam, invited the whole Muslim army in the Battle of the Trench to Abu Talhah's house for a meal, although Abu Talhah invited the Prophet, sallallaahu ‘alayhi wa sallam, alone. The Hadeeth is reported in Saheeh Al-Bukhari and Saheeh Muslim.

Ibn Battaal commented on that Hadeeth saying, “In fact, the Prophet, sallallaahu ‘alayhi wa sallam, asked the permission of the butcher who invited him in the other occasion because he, sallallaahu ‘alayhi wa sallam, did not have the same rapport with him like that he, sallallaahu ‘alayhi wa sallam, had with Abu Talhah . In the incident of Abu Talhah, the Prophet, sallallaahu ‘alayhi wa sallam, simply invited people who were with him to Abu Talhah's house for the meal (to which he, sallallaahu ‘alayhi wa sallam, was invited by Abu Talhah).”

Moreover, Al-Haafith Ibn Hajar wrote, “As for the incident of Abu Talhah when the Prophet, sallallaahu ‘alayhi wa sallam, invited people over for a gruel meal as underlined earlier in the chapter of ‘Alaamaat An-Nubowwah (signs of prophethood). He, sallallaahu ‘alayhi wa sallam, said to the people who were with him then, 'Come with me (to Abu Talhah's house).’ Al-Maaziri commented, ‘Perhaps the Prophet, sallallaahu ‘alayhi wa sallam, had known that Abu Talhah agreed on inviting people over, but in the other incident he, sallallaahu ‘alayhi wa sallam, did not know about Abu Shu‘ayb's approval of the invitation and this is why he, sallallaahu ‘alayhi wa sallam, asked his permission first …” [Fat-h Al-Baari]

Allaah Knows best.