So it sounds like you are looking to add one or more additional roommates to an existing household (that already paid a broker's fee, presumably, when you first moved in).

Landlords like to use brokers because brokers are allowed to do certain background checks that landlords cannot (it's a stupid loophole in the law that was meant to protect tenants but instead incentivizes this behavior). It's possible your landlord really just wants the background check done.

I would assume that this means that the incoming tenant would be responsible for the fee, rather than those of you who already live there. But in this case it might be best to talk to the landlord to figure out how to avoid this since you're already known tenants with presumably good history of paying on time, etc.