If you have a group of people that you want to invite to your app or send an important update to, you can do so a number of ways:
To share your app via email or to send an update to a list of email addresses via Yapp, go to your dashboard and click on "share" below your Yapp ID. In the pop-up that you see, enter the email address or addresses in the "To" box. Then enter your "From" name and "Subject" and add an "Optional Message".
To add multiple email addresses at once, copy/paste the emails from a .csv spreadsheet or enter them separated by a comma, or by pressing enter after each email address.