Under the Apprentices section, click on the orange add button in the top right hand corner.
A menu will pop up allowing you to enter the details for your new apprentice.
When you scroll to the bottom, you'll see a checkbox that says Do Not Save. Check the box and the Invite button changes to a Save button. You can now save and add your apprentice into the system, and invite them at a later date!
When you're ready to invite them, find their account. At the top, you'll see a yellow box stating the apprentice has not joined yet. Click Send Invite to invite your apprentice!