A sole proprietorship is no different than any other account owned by an individual. If the bank allows agents, convenience signers, or authorized signers on other individual accounts then there is no reason not to allow them on a sole proprietorship account.
They are signatories, not owners; the term "joint" does not apply. The owner can add them or take them off at will.
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In this world you must be oh so smart or oh so pleasant. Well, for years I was smart. I recommend pleasant.