If you are a super user you can see a list of users by clicking on the Account Settings option in the dropdown by clicking your name in the upper right hand corner of the page. From there you can select Staff or Student and view the active users of each type. You can also Export a user list to a CSV file, view Deleted users or add new ones.
You can see a list of Super Users by clicking Super Users under the Staff section.
Teachers can see a list of students in each class by clicking on the class folder and then the roster tab. Depending on account settings, teachers can edit the roster by adding or removing students as members of the class.