1. Go to the course where you want to invite participants into. 
  2. Click on the 'invite potential participants' button: 
  3. Enter the email addresses of the participants you want to invite (1). When you want to add more participants, click on 'add potential participant' (2). Also fill in the deadline, this means that a slot will be reserved for every invited participant until the entered deadline (3). Then click 'Save' (4) : 
  4. You are now able to see the participants you've invited (red arrows on screenshot):