Unless your state law has something to say on the question (some states have specific laws addressing "convenience signers" or "authorized signers"), it's generally up to the bank to set up its own restrictions.
One restriction I do recommend: for a joint account, any authorized signer that is not an attorney-in-fact for one of the owners must be approved by all joint owners. And if an attorney-in-fact for one of the owners has to be added, I recommend a procedure that notifies all owners of the addition.
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John S. Burnett
BankersOnline.com
Fighting for Compliance since 1976
Bankers' Threads User #8