Is what the questioner does—taking 10% of his friends' profits in exchange for investing their money along with his own and providing the service of bringing the profits to them—considered permissible or forbidden?

It is permissible to take money from these individuals to invest it with another person, with their permission. The profit is then divided according to the agreement. Among the conditions of Mudarabah (partnership) are that the share of both parties from the profit must be added to the profits, and that the Mudarib (investing partner) does not guarantee the capital except in cases of transgression or negligence. A condition for joint Mudarabah is that the capital contributions must be clearly distinguishable.