I appreciate the help, but again, that does not answer the question. I know how to set up the workflow. What I need to know is why a particular person who will be responsible for bill approval does not show up in the list of users when I try to set it up. 

What permissions, specifically, does a user need to be a bill approver?

And while we're at it, what permissions, specifically, does a user need to be able to schedule bill payments? The user is a secondary admin, has full rights to pretty much everything, and is an admin in Melio. Yes, she still cannot schedule bill payments. The error says that she doesn't have permission.

There should be a guide somewhere about activities like this and what permissions need to be set to turn them on.