To re-send just one user's login information email out to them again, click on the People tab, then click in to the person's profile, and then click the red 'Send login email' link on the right side of the page. This will re-send only their login details out to them again.
If you ever want to re-send all of the login emails out to every person that has never logged in to their account, then you can do this by using the 'never logged in' link on the main Admin Dashboard or the 'Send login emails' link on the People tab.
Note: Once someone has logged in you cannot re-send login information. You can only "Reset" their password.