You can send a call invitation to anyone's email from within the Highfive application:
The invitee should receive an email from Highfive within a few minutes.
If for some reason the invitee doesn't see the email in their inbox within a few minutes, have them look in their Spam or Trash folders to make sure that their email client isn't hiding it.
In some extreme cases, corporate anti-spam tools may quarantine invites before they ever reach the intended recipient. In those cases, you'll need to contact your IT Help desk for assistance.
You can always copy the link and paste it into some other form of communication (IM, text, email, even verbal) to avoid any spam filtering issues.