I can add a user to the Active Users, but I can't add the user to the Members group on my site

I'm using SharePoint 2013 on Office365.

I've added two users in the Admin Portal, both with identical attributes (except name, email address etc.!)

However, when I go onto my site and try to add them to the Members group, only one of them is available. The first image shows adding the user which ICAN add:

This shows the one that I cannot make appear:

If I complete the "unsuccessful" adding of the user, then the user receives an email inviting them to the site, but they don't appear in the list of Members:

I'm at a loss to understand what I'm doing wrong.  The "successful" user Sam Wilson has exactly the same attributes as the "unsuccessful" user, except that SharePoint can find him in the list of users in the SharePoint users portal.