If the user you invited has not received the invitation email, you can do a couple of things:
1) Ask the user to check his/her Spam Folder. Sometimes the email goes into Spam Folder due to the user's email server or mailbox configurations.
2) If the email cannot be found in the Spam Folder, it is likely that the email was bounced or undelivered. In such case, you can find the email content from the web console and copy it into an new email and send it to the user using your email program.
2a) Go to "Manage" >> "Manage Users" >> "Invited Users" page.
2b) Locate the "View Email" link for the respective invited user.
2c) Click the "View Email" link to bring up a dialog that shows the content of the email.
2d) Copy the email body into a new email message in your email program and send it to the user.