1. Get the email ID of the person you need to send the Google Plus invite.
2. Post a status update or if you have posted a update earlier then you can use it too.
3. Share that status update or anything on your profile with the person you need to send the invitation too. Enter the E-Mail of the person in order to share the same.
4. Now the person will receive your invitation as an E-mail. He/She can follow the link in order to sign in to there Google Plus account.
via Get Google Plus Invitation & Learn how to send Google Plus Invitation.
