The instructor is automatically the "owner" of a list. The owner can add "collaborators" who can also work on the list.
The Collaborators area is on the right pane of Leganto. Click the Collaborators link to see who is currently assigned a role in the list. Click "Manage Collaborators" to add/remove them or change their privileges.
To add a collaborator, enter a name into the "Invite new collaborators" box. Select the option that matches the privileges you want them to have:
Click "Send invitation".