How do I create the rosters for my classes?
If you haven't checked out Dashboard Help already, here is the link: https://dashboard.duolingo.com/dashboard_help , the information under the header "How do I get my students started on Duolingo?" seems relevant to what you are asking.
I hope I am understanding your question correctly (how to create/sort your students into different classrooms, i.e. creating your rosters), I'll ask that you forgive me and perhaps clarify your question if I have misunderstood you.
When you are at your Dashboard there is a button on the right hand side of the screen that says "Create classroom". After clicking it you should get a prompt asking you to name the class/section and select the language that you are teaching. Save your changes and it will add an empty classroom to your Dashboard.
If you click into that class there will be a blue button in the upper right corner that says "Invite students". From there you can generate a unique link that you can give to your students to add them specifically to that class. This classroom will populate as your students accept your invitation to monitor them.
You can add multiple classrooms to your Dashboard to sort your students by language, language level (Spanish 1, Spanish 2 ...), period/section, etc. If you have any wrongly assigned/unassigned students you can also move them into or between classes as well as merge classrooms should you desire to do so in the future.