It is prescribed to end a gathering by saying that which was narrated by Abu Barzah al-Aslami (radi Allahu ánhu) who said: The Messenger of Allaah (sallAllahu ályhi wasallam) used to say, when he wanted to leave a gathering
سُبْحـانَكَ اللّهُـمَّ وَبِحَمدِك، أَشْهَـدُ أَنْ لا إِلهَ إِلاّ أَنْتَ أَسْتَغْفِرُكَ وَأَتوبُ إِلَـيْك

“Subhaanaka Allaahumma wa bi hamdika, ashhadu an laa ilaaha illa Anta, astaghfiruka wa 'atubu ilayk"
 
Glory is to You, O Allah, and praise is to You. I bear witness that there is none worthy of worship but You. I seek Your forgiveness and repent to You.
And he said: “It is a 'kaffara' [expiation] for whatever happened in that gathering.”

 
(Abu Dawood 4859 - Classed as saheeh by al-Albaani in Saheeh Abi Dawood, Ibn Majah, At-Tirmithi and An-Nasa'i. See also Al-Albani, Sahih At-Tirmithi 3/153)