In the blud admin bar, hover over the "People" dropdown and select "Groups". You'll taken to the Groups search view.
Click on the Group's name and you'll be taken to that Group's page:
Next, mouse over to the right-hand side of the screen and click the "Email all x members"
From the email screen, you can draft and send your email to the group members. This is NOT to take the place of a newsletter. It is not meant to function with a large volume of users that a newsletter might require.
Before sending your message, be sure to uncheck the "Send test email to me only" box. Leaving it checked will only send an email to you. Unchecking it will enable all group members to receive the email.
It's alright to leave the "To" and "From" fields unchanged. Create a subject line and body and click "Send Message".
With that, you've just sent an email to a user group!