Is the questioner considered a guarantor of his friends' money with the merchant, given that he was an intermediary for them or encouraged them to trade, with his prior knowledge that it was a trade in recharge cards, and it later became clear that it was speculation in the stock market?

The Mudarib (entrepreneur/agent) is an amin (trustee) and does not guarantee against capital loss unless there is transgression (ta'addi) or negligence (tafrit). Examples of transgression include speculating with the Mudaraba capital through another party without explicit permission from the capital owner, or speculating in ventures not specified for him. If the Mudarib violates the agreed-upon conditions and the funds are lost due to his violation, then he guarantees the loss.

Therefore, the Mudarib mentioned in the question is liable for the capital loss due to his transgression by placing the funds in risky speculations (stock market) without permission. He must return the capital to its owners. As for the questioner, he is not liable to his associates because he is merely an agent for them in delivering the money to the Mudarib, and he did not deceive them.