We recently had a customer open a Business MMDA and wanted to add a beneficiary to this account. I was under the impression that if it is a sole proprietor you can add a beneficiary otherwise corporations, LLC etc. can not. Any help on this would be appreciated.

Your instincts are good. For all practical purposes, a sole proprietor's account is an individual account and some banks allow the naming of a beneficiary. However, a corporation, LLC or partnership is not going to "die." The concept of survivorship does not exist on those types of accounts.