Yes I think your concern is entirely justified if it is a joint account. It could also create a tax nightmare. Let me guess that the broker suggested a joint account to you because they are not sure that they fully understand the relationship between you and the trader. So the broker figures he will just protect his firm by getting everyone's name on the account. Brokers are fearful that there are hidden parties to the account not listed on the form. This gets them in trouble with regulators. So some brokers may be reluctant to let you designate the guy as a only a trader unless he is a CTA or whatever...... I guess if that is the case you could just give the guy your id and password. It is not ideal .....
Couldn't you just set up an llc? He be your trader and you be the "owner" of the llc. I don't know anything about the tax implications but I imagine that is a possiblity too although I think you probably want a certain amount of capital to make that a good option. GL.
Many brokers, including IB, allow one to open an account in the ownership form of their choice (e.g., individual, corporate, LLC) and provide access to a non-owner for specified purposes. For this type of set up all liability for losses resides solely with the owner and all tax reporting is prepared with the owner's taxpayer ID. At IB, legal access to the non-owner is provided via a power of attorney agreement delivered electronically. Here the owner can specify what account-related permissions he would like the non-owner to have, including trading and statement review. In no event will we allow the non-owner to have access to funding activities such as creating banking instructions or initiating withdrawals nor will we allow the non-owner to change the owner's account information (e.g., email, phone number, mailing address), the products/markets the owner has requested to trade or margin permissions. As a technical matter, these permissions and restrictions are controlled through issuance of log in credentials (user name and password) separate from those of the owner. It is through these log in credentials that the non-owner is also able to contact and receive assistance from Customer Service but only for matters relating to those permissions which have been granted. Providing permissions to a non-owner does not preclude the owner from exercising those same permissions (e.g., both can trade) and the owner has the ability to electronically revoke or amend permissions granted to a non-owner at any time. One item of note, however, is that the owner and non-owner cannot share the same market data subscriptions. Each user has the opportunity to subscribe to the data of their choice for streaming to the trading platform, the costs of which will be charged to the owner. If you have any questions or require assistance with this feature, feel free to PM me.