What if somebody has lost their email invitation? Can I invite them again?

Yes, if that person has not yet accepted their invitation you can invite them again. It will email them a fresh invitation. Or, they can simply self-register on your community and that will also give them access, as your invitation will act as a pre-approval of their registration.

So it's a good idea to have the registration link or login button (available from the Setup Wizard) available somewhere prominent on your school or PTA website, Facebook page etc.