@sarnold Turning your comment into an answer because it worked for me.

Two Separate Websites

Apparently for historical and organizational reasons, Apple operates their developer and app store business in a bifurcated manner.

• developer.apple.com

  The web site for all the technical resources, including documentation, WWDC videos and such.

• itunesconnect.apple.com

  This completely separate web site handles the business end:

  • Contracts, banking, payments.
  • Uploading your finished app.
  • Defining your In-App Purchase products.

Problem: Different Handling Of Accounts

The developer.apple.com site is slick in that a programmer can have a single Apple ID used to join multiple developer accounts. She may work for three different companies, and be assigned a role on each of those companies’ Developer accounts all on the same Apple ID. When she logs in to the Developer site, she is presented with a popup menu asking which of the three companies’ Developer accounts she wants to access during this work session. Nice.

The problem: in iTunesConnect, not so nice. In iTunesConnect, the “admin” person cannot assign existing Apple IDs as members with a role. Very strange. The admin is forced to create a new ID for each person being adding to the team. That means the person joining must have multiple email address. If an admin tries to add you to their iTunesConnect with an email address already used on some other company’s iTunesAccount, an error message reports that email address / Apple ID is already in use. Obviously the programming team running the iTunesConnect site could use some help from the Developer site’s team. ☹

Workaround: Email Address Trick

The workaround cited by sarnold involves a feature of email addresses. The spec for email servers has a feature where you can extend your email address. You can add a suffix to the first part of your email name by appending a “+” PLUS SIGN. From what I could figure out, the email software first looks for the extended name. If no such name is found, it drops the extension and looks again. If found, the shorter version of the email address is actually used.

So if the programmer Susan wants to use her email address susan@example.com for a second or third iTunesAccount, she tells her client’s admin person at Acme Corporation to use something like this as her email address: susan+acme.com@example.com. Apple will still create a needless extra Apple ID for Susan, but at least Susan does not have to bother with creating and accessing extra email accounts. The emails sent by Apple will arrive at her susan@example.com address.

Susan double-checked this would work before talking to the admin person. She sent herself an email to susan+acme.com@example.com to make sure it arrived at the susan@example.com account.

Effectively, the email servers are not fooled by the extended email address, but iTunesConnect is fooled into creating an new Apple ID using an old email account.