Users are assigned to groups, which can be done in the groups tab, where you can add existing users or create users for that group from scratch.
Start by heading to the groups tab.
Select the group you would like to assign a lesson to.
This will take you to the group management page, where you will be able to see each aspect of the group.
From here, select the lessons tab, and you will be able to manage lessons for this group.
When you are ready to add a new lesson, just click the green "Add Lesson" button and
select the lesson you would like to assign, then pick a start date and you are good to go!