They will need to click the “Accept Invitation” button in the invitation email. If they are logged into their WordPress.com account, they will be taken directly to your blog. If not, they will be taken to the log-in screen, where they can either log in or create a new user account.
Note that users who create a new WordPress.com account at that point will still need to return to the invitation email and click the “Accept Invitation” button after doing so.
Viewing and Removing Viewers from a Private Blog
You can see who has accepted an invitation to view your private blog as well as remove viewers by going to your Dashboard and navigating to Settings > Reading. You will see a list of viewers in the Site Visibility section: