Valid for Moodle 2.8. Updated on 5/26/2015.
For this to work, the Groups setting for the course must be set to visible or separate. This is set by changing the course setting.
If a course requires students to enroll in a co-requisite course, such as a lab course that is required by a science course, emailing the members of a particular co-requisite is a minor variation on the general procedure for emailing students. In this situation there will be an additional option in the View as List box on the right.
Under the Separate or Visible groups list you can pick one of the co-requisite course sections. Once you've done that you can click on the Select all link and then With selected users... send an email using the pull-down menu.
There is no simple way to email some, but not all, of the co-requisite course groups.