It is narrated from Sayyiduna Abu Hurayrah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "When one of you is invited and comes along with the messenger, that is his permission."
The Shaykh says that if there is no one whose privacy must be guarded (in the house), then this is permission; otherwise he should come to the door and seek permission again, and this applies after the revelation of the verse of hijab.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الأشربة والحد فيها / 17672