Shared Rosters
Shared rosters allow a school to designate that the exact same set of students should be in more than one class together. This can be used to ensure continuity of rosters across terms (e.g. "all students in Period 1 Visual Arts 1 in Semester 1 should be enrolled in Period 1 Visual Arts 2 in Semester 2," or across multiple classes in the same term ("Students in Mr. Gonzalez's 2nd period English class should be in Ms. Chu's 3rd period History class").
In order to be able to designate two or more sections as having shared rosters, the sections must:
Be scheduled to a period and term
Have at least one teacher assigned
Have an empty roster
Linking Rosters
Open the Courses board to view sections

Open the section inspector by choosing a course and selecting a section
Under the Students section, toggle Shared Rosters to on

In the Search for a Course search bar, type the name or course code of the section you wish to link and select the course from the list of suggested matches

The list of sections of the course you have selected will appear - select the class that should share rosters with the current section
The two sections' rosters are now shared. When you run a tumble, students enrolled in one section will be enrolled in the linked section as well.