Once you click save, the user you've added will receive an email with a link to set their own password.
If the user you're adding already has an account, they will be sent an email informing them that they have been added to access this new account, and you will not need to add their name when adding their email address.
N.B. The password setting link expires in 3 days if it hasn't been used. If you need to send a new password setting email, you can enter any email address to the 'Forgot my password' page.