Updated for the new SurveyStance app: you now assign devices while inviting the user. Go to Account Settings > Users > Invite User, check Survey Kiosk under Product Access, and choose All kiosk devices or Specific devices. For existing users, click Edit on their row in Account Settings > Users. See User Management: Add or Invite Users.

When you add or invite sub users, make sure to assign a device to that user.
STEP 1. Click on Devices on the sidebar, locate the specific device you want to assign to user and click the Blue Pencil Icon to Edit.
STEP 2. Click the 'Assigned To' and select the users email address. Click Save.
To assign a device to a new sub-user: Survey Kiosk Module -> Device & User -> Devices -> Edit -> Assign to: input your sub-users name