If you invite someone to your organisation (or someone else does), you'll see their name in the list on your Organisation page. You may find that they lose or delete the invitation, or that it gets lost in your organisation's spam filter. Because no-one can create an account without an invitation email, you may thus need to resend it.
Visit your Organisation page and find the list of invited users by scrolling down.
Find the person you're interested in in the list, and click the green envelope on the same row. This will resend their invitation email – you'll see a green notification at the top right of the window to show that this has been done.