To send an email to all of your users, or a short list, enter the Members tab and search for the users you want to email. (If you want to search for all users in the data room, simply perform a blank search). Once you are happy the correct people are listed below press the Send Email button.
Your default email client (Outlook, Thunderbird, Mail) will open and automatically enter the email addresses of the users into the bcc field. Then enter your message and press send.