When you invite a designated reader to your private blog, or invite a member to your private or team blog, you use the Settings - Permissions wizard to send an email to the prospective member or reader. You select the email address to send the invitation to, based upon the known address of your prospective member or reader.
The prospective member / reader, upon receiving the email, is free to send it on, to any other email address that he or she uses, and to accept the membership by using any Blogger account - current, or setup at the time of accepting the invitation.
This allows anonymity in membership, by permitting somebody to accept membership, period. You have no way of knowing, nor should you know, what account or what email address your new member uses to accept membership. The only thing that you know is that a membership invitation was accepted, by a given prospective member.
Unfortunately, just as you don't know what account the membership was accepted under, you have no way of knowing how many people other than your targeted member were sent a copy of your invitation. That's a possible reason for some blogs mysteriously exceeding the 100 member limit.
The bottom line here is, just as you must choose your blog administrators wisely, so should you choose your members and readers. Don't just send an invitation to somebody, without considering the possibilities.