Also known as a conservatorship, a guardianship occurs when an individual is no longer able to make sound financial and legal decisions on their own behalf. When a guardianship occurs, the guardian themselves do not take hold of the assets, they merely control the assets. The assets still fall under the individual’s own SSN/ITIN, so no additional tax ID or EIN is needed. The guardian will not use their own.
However, there is a reason why a guardianship could need a tax ID/EIN. That’s if the individual who is under guardianship has a trust account. If a trust account is opened for the individual, and if that trust account is irrevocable, then there may need to be an additional tax ID/EIN to hold those assets for the individual.
Otherwise, the bank accounts will be under the individual’s SSN/ITIN with the conservator/guardian as a co-signer on the account, and the guardian will be able to control financial accounts through agreement with the financial institutions rather than taking them under their own SSN. It’s important that the guardian not take direct ownership of the funds as they can be called into question or they can be liable for taxes.