There are no email attachments sent from the ZoomIn website when you share an Album. The pictures your friends see are securely stored in the ZoomIn computer system, and they are viewed on the ZoomIn website.
When your friends receive the share invite email from you, they will click on a link that brings them to the ZoomIn website. If they don't already have a ZoomIn account, they'll be asked to create one by providing their email address and selecting a username and password. From there, they will view only the photos you have chosen to share with them.