Is it permissible for an agent to take a commission (brokerage fee) of up to 5% from a fund management company for bringing in investors, with some investors being aware of this and others not, and for part of this brokerage fee to be shared with the agent himself?

If the commission is paid from the company's funds, there is no objection. If it is paid from the investor's money, his knowledge and consent are required. In both cases, there is no objection to the client and the representative sharing this percentage by mutual agreement. However, taking a commission from the investor's money without his knowledge is forbidden and a grave sin. The investment must be in what Allah has made lawful, without usury, forbidden sales, or assisting in what is unlawful.