If you have sufficient rights (you are an admin user or a teacher/staff member), after creating a group you will see a link called “Add by Class/Grade” in the Members area.
Clicking this link will bring you to a page where you can select a grade or homeroom. Once the students associated with your selection are displayed, simply click the checkboxes next to those you want to add to the group and submit.
The group will be redisplayed with all of the selected students listed as members.