Duas for sitting in a gathering. Fill the sitting with forgiveness, and close it with glorification
The Prophet ﷺ would ask forgiveness many times in a single sitting, and he closed his gatherings with words he described as an expiation for whatever was said in them.
2 sourced Du'as Arabic, transliteration, meaning, and source
Subhanaka-llahumma wa bihamdik, ash-hadu an la ilaha illa ant, astaghfiruka wa atubu ilayk.
“Glory be to You, O Allah, and all praise is Yours. I bear witness that there is no god but You. I ask Your forgiveness and turn to You in repentance.”
Jami at-Tirmidhi 3433 (sahih), Sunan Abi Dawud 4859 (hasan sahih)
Said before rising from a gathering. The Prophet ﷺ called it an expiation for whatever was said in it.
عَنِ ابْنِ عُمَرَ رضي الله عنه قَاَلَ: كَانَ يُعَدُّ لِرَسُولِ اللَّهِ صلى الله عليه وسلم فِي الْمَجْلِسِ الوَاحِدِ مِائَةُ مَرَّةٍ مِنْ قَبْلِ أَنْ يَقُومَ: رَبِّ اغْفِرْ لِي، وَتُبْ عَلَيَّ، إِنَّكَ أَنْتَ التَّوَّابُ الغَفُورُ
'An Ibn 'Umar radiya-llahu 'anhu qal: kana yu'addu li-Rasulillahi sallallahu 'alayhi wa sallam fil-majlisil-wahidi mi'atu marratin min qabli an yaqum: Rabbighfir li, wa tub 'alayya, innaka antat-Tawwabul-Ghafur.
“Ibn 'Umar said: In a single sitting, the Messenger of Allah ﷺ would be counted saying a hundred times before he stood up: My Lord, forgive me and accept my repentance. Truly You are the Accepter of repentance, the Forgiving.”
Jami at-Tirmidhi 3434 (sahih), Sunan Abi Dawud 1516 (sahih), Sunan Ibn Majah 3814 (sahih)
The du'a itself is Rabbighfir li wa tub 'alayya, innaka antat-Tawwabul-Ghafur. Abu Dawud and Ibn Majah end it with at-Tawwabur-Rahim.