What is the duty of someone who committed immoral and indecent acts with his companions while they were asleep, and how can he expiate his sin, knowing that he does not remember all of them and does not dare to inform them of this?

Sincere repentance (Tawbah Nasuh) and seeking forgiveness (Istighfar) are obligatory, along with the firm resolve not to return to the sin, and to increase in acts of obedience and associate with righteous people. You are not required to expose yourself; rather, praise Allah for His concealment, repent, and seek forgiveness for them and for yourself.