Invited as external user for Office 365: Access to a site as an external user
Now be careful when you click on the link, there’s a complex behavior there. Depending on what’s open on your browser or if you already have a Microsoft account or not, you might link the invitation with a wrong sign in authentication.
Using the SharePoint Online Management Shell, IT administrators can now require that all external sharing invitations be accepted only by the e-mail address that was originally shared with.
To enable this feature, simply run the Set-SPOTenant cmdlet and set the “RequireAcceptingAccountMatchInvitedAccount” parameter to true.
The parameter accepts two values: True or False.
True-User must accept this invitation with firstname.lastname@example.org.
False– When a document is shared with an external user, email@example.com, it can be accepted by any user with access to the invitation link in the original e-mail.
Administrators who desire increased control over external collaborators should consider enabling this feature.
|This only applies to new external users accepting new sharing invitations.
The resource owner must share with an organizational or Microsoft account or the external user will be unable to access the resource
Click additional information to learn more. https://technet.microsoft.com/en-us/library/fp161390.aspx