In Admin > Comms on the left at the bottom of the groups there is an option called by attendance. If you click this it will expand and give you the choice of different codes:
Click AM - Y code or PM - Y (or any of the other codes) code to see those students who have had the particular code assigned that session.
You can then click individual students to add them to the contact list to send out Comms to or use the 'Add all from AM - Y (or any other code) code' option to add all the students. They will then show in the contact list ready for you to send an email or sms