Click on the user's profile button from within the property profile. You can also access their profile from the Contacts tab.
On the Edit screen, click Send Invitation. If you have already sent one before, this will be a Resend Invitation button.
You can always check the status of a user's profile by the user icon - if it has an exclamation mark, they are yet to accept their invitation.
Hint: Make sure the email address is correct, as they won't be able to log in if there is a typo or if it's wrong.