Typically we require a partnership agreement. For those foolish enough to operate without one, we usually ask for a business license, a TIN/EIN letter form the IRS, and copies of tax return schedules ahowing the partnership name and the partners involved in the business. I want to say that info is on a K-1, but don't quote me (it's been a long time since I've reviewed tax returns). I think there is also a schedule in the business tax return that lists the partners' names.
If they don't have that, their only other option is to open as a sole proprietorship and list the extra people as signers.
I can't herd the cats anymore, so I just set up the electric fences and let them fry when they stray out of bounds.