Is it permissible for some preachers to recite aloud the supplication for expiation of the مجلس (gathering) after sessions of dhikr, with those present repeating after them, or is the Sunnah for each person to say this supplication individually?

The expiation of the مجلس (gathering) is prescribed after gatherings of remembrance (dhikr), and it is: "سبحانك اللهم وبحمدك، أشهد أن لا إله إلا أنت، أستغفرك وأتوب إليك" (Glory be to You, O Allah, and with Your praise, I bear witness that there is no god but You, I seek Your forgiveness and repent to You). As for preachers mentioning the expiation of the مجلس and people repeating it after them, if it is for teaching, then there is no harm in it. However, if it is a permanent method of worship, we do not know of any legal basis for it; because "Whoever introduces into this matter of ours that which is not from it, it will be rejected."