I recently joined Google Plus after receiving invitation from a friend. Google plus is currently under beta phase and you can join Google plus only via invitation. However, Google plus teams says it will be open for all soon and you can join Google plus without any invitation.
An already member of Google plus can send unlimited invitations to friends / family members etc. Only requirement to join Google plus is an invitation and a Google Account.
If you are a member of Google Plus, here is how you can send invitation to others.
If Anyone need Google Plus invitation, please leave a comment below with your Gmail / Google Account ID.