Make sure these settings are set:
TENANT WIDE SETTINGS
This is already set for our Sharepoint Tenant.
Move to SITE SPECIFIC SETTINGS.
SITE SPECIFIC SETTINGS
Leave all the rest as "Same...."
ADD THE CONTACT TO AD 1ST
First add the new person as a Contact using their personal email address. Make sure to place the in the correct OU.
NOW SHARE THE SITE TO USERS