Valid for Moodle 2.8. Updated on 7/20/2015.
The simplest way to email a section is to use the section course site and email the participants in that site. Even if you don't use the site for anything else and don't make it visible to the students, you as an instructor can still use its mailing ability.
There is no easy way to email multiple (but not all) sections at the same time. You either have to repeat the mailing in each of the section sites or select the students you want to email using the participants page in the umbrella course site. If you do this alot you may want to create a group to send email to. That involves selecting all the students for the group and then using the With selected user pulldown to create a group. 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.
Note that individual sections are different than co-requisite courses such as labs connected with science courses. Mailing members of co-requisites can be done from from the participants page of the umbrella courses.