VS Marketplace Add Member displayes Invalid Domain error

I'm having the same issue with a newly registered Publisher and have contacted Microsoft. Here's what I was told:

Apologies for the inconvenience you are facing. This is currently due to an issue at our end. To unblock you, we can add the users manually to the publisher. Could you please provide us the VSIDs of the users that you are trying to add to the publisher?

To provide the VSID, do the following:

1. Login to the Marketplace (https://marketplace.visualstudio.com) with the email address that has to be added to the publisher.
2. After getting logged in, from the same browser window, open https://marketplace.visualstudio.com/_apis/connectiondata
3. You should see some JSON data in the window.
4. Copy paste the output of the page that you see. We are interested in the "id" field of the "authenticated" user.

We will manually add the users to your publisher, once we have the VSIDs details.

You'll want to contact Microsoft using the "Need help? Contact Microsoft" link that Grzegorz Smulko also mentioned, but you might as well collect the relevant IDs to include when you reach out to expedite the process.

Microsoft may be requiring Microsoft federated(Hotmail, Live..., or AD integrated) IDs.

Due to the complete lack of documentation, I have been unable to verify this claim.

This could also be attributed to Microsoft banning gmail.com as a valid source e-mail domain.

There is an alternate way to add members via User Id. Please follow the steps mentioned here:

1. Ask member (that you want to add) to login their account on Marketplace
2. User Id will be visible by hovering over email as shown in image.
3. Member can copy the User Id by clicking copy-to-clipboard button and share with the current owner of the publisher(you).
4. Now the current owner of the publisher(you) can add member using User Id from members tab. Click on + Add -> Now enter the User Id-> Select a role you want to assign to the new member.

As shown in the below screenshot.

It looks like a bug to me.

I've tried to add a new member from the same domain as all the other existing members and I'm getting the very same error.

I'd suggest contacting MS using the "Need help? Contact Microsoft" link from the top-right corner.