If you've enabled and created multiple sections in your course, you'll most likely have different students in each section. This means you'll need to assign students to a section, unless you invited them to a specific section in your course.
Once you've invited your students, you'll be able to assign them to a section. You can also assign instructors to a section in a course (if you've invited another instructor who will be teaching a section of the course).
To assign users, start by navigating to your course home page and clicking on Roster on the right sidebar.
To assign users to a section, click on the circles next to their names, then click on the Change Section button.
You'll then see the names of the sections in your course. Click on the section you'd like the users to be in and they will be assigned to that section.
You can also click on the Filter Sections button and click on the circle next to one of your sections to see which users have been assigned to it.
Once you've assigned users to sections in your course, you'll have the option to set start and end dates for assignments by section.