You can add a new class by logging in to your account, clicking "Actions", and selecting "Create a New Class".
Each of your classes needs a unique name to help you tell them apart.
Select the grade level of the class from the dropdown menu. This will be the starting grade for all students added to this class. However, you can change any individual student's grade to give them a different experience. You can see this article for instructions - How do I change the grade level for one of my students?
Schools that roster have classed created and populated when the files shared with ST Math are uploaded. Because you can change any individual student's grade to give them a different experience while keeping them in your rostered class, students with different grade level needs can be kept in the same rostered class.