I understand if all accountholders are non U.S. persons they should each sign a W-8BEN. I also understand that if one is a U.S. person and the others are not, the U.S. person should be placed first on the account and would certify the W-8, however, would it be necessary for the other non U. S. accountowners to sign W-8BENs?

It is my understanding that anyone on an account must have the W-8 on file. If you do have a U.S. person it would be smart to put them as the primary. As a precation to cover ourselves we also put a slip with the W-8 that they sign that says the are responsible for any fees the IRS may charge us because they don't have a social security number. I don't know what kind of charges that would be, but I think it is a "just-covering-our-butt" sort of thing.
Sorry, I don't have any documentation on that, just our policy.

If a US Person is a joint accountholder on an "interest bearing" account, you would be required to have a W-9 for that person. A W-8 would not be required on the non-US Person joint acountholder. If this is non-interest bearing account, you are not required to obtain either a W-9 or W-8BEN from either party.