All perfect praise be to Allah the Lord of the Worlds. May His peace and blessings be upon our Prophet Mohammad and upon all his family and companions.
Based on your question, it seems that the fund participants aren`t obliged to get rid of the ill-gotten portion of these profits and they aren`t considered sinful. This is because of two things:
First: The unlawfulness of investing these funds is not clear, as evident from the question, and since it is unconfirmed, then, in principle, it is lawful and permissible.
Second: You (Employees) weren`t aware of the details of the investment method and a Muslim isn`t obliged to examine every step that his partner does regarding the investment. However, he should demand the latter to be keen on making lawful money and avoiding suspicious matters in general. If he abides by this piece of advice, then thanks to Allah, but if he doesn`t, then he is the one liable before Almighty Allah. And Allah the Almighty knows best.