If you are a registered Google+ user you can now invite your friends, family, or whoever you like.
to invite someone you can go into your Circles from the Google+ top button area, click on Add a new person, enter an email address and choose a circle for this person to go into. Once you click Save, a bar along the bottom will appear with a green button marked Invite to Google+ – click this button and an invitation will be sent via email.
During the time where Google+ invites were deactivated, there were several methods to still invite people and bypass the Google-initiated block. A popular one used as Neowin report was to enter a specific email address as a person to share a post with; the person was then automatically created a Google+ account and via the email sent, could log in. As Google has re-opened the invites, this should no longer be necessary.