On a class's page, select Add students. You'll see up to two tabs, depending on what's available:
New students - paste one name per line, or upload a CSV (a name column is enough; anything else is ignored). You don't need to set a year level here - every student created this way automatically inherits the class's own year level.
Move from another class (labelled this way, rather than just "Existing students") - lists every student already on your account who isn't currently in this class, for moving someone between classes without recreating their profile or losing their history.
Preview what you're about to add, then confirm.
Giving each student a PIN
Once students are on the roll, each row has a Set PIN button (or Reset PIN, if one's already set). This works exactly like a family's Kids Portal PIN: shown to you once at the moment you set it, offered as a printable sign-in card, and never displayed again after. See How students sign in for how a student actually uses it.