You may use a financial power of attorney to grant a trusted agent authority to act in your place in financial matters. The terms of this legal document describe the extent of the authority it confers. While a general form would allow your agent to add signatories to your bank accounts, you can tailor the authority as you wish.
Financial Power of Attorney
A power-of-attorney form allows you to appoint someone to make decisions in your place. For example, a financial power can give someone authority to sell your car or your stock holdings. Additionally, you can use a durable financial power of attorney to name the person you want to manage your financial affairs if you should become incapacitated.
Scope of Authority
Preparing a power of attorney is a personal option, not a requirement. You can define and control the authority given by writing your limitations clearly into the document. If you sign a general-power-of-attorney form without including limitations, you give the agent authority to take any action regarding your finances that you yourself could take. That would include adding signatories to a bank account.
Your agent acting under a power of attorney owes you a very high duty of care called a fiduciary duty. This requires him to act in your best interests and prevents him from taking steps to enrich himself at your expense. If his decisions regarding your bank accounts violate his fiduciary duty, he is subject to civil and criminal penalties.