If you had made account on Google+ and want to invite your friends to Google+. Here is how you can invite your friends to Google+ with their email id.
Sign in into your Google+ account
On the right side bar, under the Hangout, you will see "Send Invitations"
When you click on it, a box will open on the same screen asking for the email ids
Insert email id's, you can also insert multiple email id's to send invitation.
Click Invite, and your friends will receive invitation email, and they can also join Google+