Unable to add a returning employee to a Sharepoint Group

Hi,

I'm trying to add an employee to a site permissions group.

I can select their name but when I select Share the name does not appear in the list.
I've tried other names and they appear

The employee whose name I cannot add is a returning member of staff so I'm assuming that they have not been set-up correctly in Azure.  Would this be a potential reason?

Any other ideas why I cannot add this person???