At the top of the page, go to the 'My account' menu and choose 'Manage school'.
Then choose 'School users' and then the button on the right to 'Invite new user'.
You can then invite new users using their email address.
Your pending invites will appear in the 'school users' section until they accept your offer to join.
The new user will be invited via email. They simply have to click the link to start creating their profile.