Check it out guys, in case some of you don't know already. If the invite button on your home screen is missing here are the steps to invite more people anyway.
1. Sign in to Google+ and make a new circle called "invites" or whatever you want it to be.
2. Add people to that circle using their gmail address.
3. On the left hand side of your home page, you will see where it lists all your circles, click on the circle you just created and it should take to you a screen with no posts on it.
4. Make a new post saying "Click on "comment" to join."
5. Make sure the post is only going to the circle you just created.
6. Click "Share."
When you click "Share" it will send an email to everyone you added in that circle, telling them that you invited them to join Google+ and to click on "comment or view post." When they click "comment or view post," it will take them to the post you just made.
If it doesn't let them join right away, tell them to leave that screen up on their PC and when google allows registration again the page will automatically switch. If they don't want to leave the screen up, then just tell them to keep checking back every now and then.
