Use Group Mail when you need to email a group of people who aren't necessarily on a single team.
To do this, go to the People page and filter down to the group you'd like to actually email.
Then, click on the "Action" menu at the top-right of the screen.
From the menu, simply select, "Send email to all results". A sidebar will open that will let you start composing your email.
Just like any other email in Lineup, you can customize the From name and Reply to email address. You'll need to add a subject line for your email and the contents of the email itself, obviously. You can preview the email as well.
Once you're ready to send it out, click "Send email now" and off it goes!