My understanding was that that's only available with Google Apps for Business, but that article seems to contradict that. With that said, I don't see the option to enable Google+ on my free Google Apps account.
Works for me.
It's in additional services. The sorting places it after everything else that starts with "Google"
I don't see it there, but I probably just have it disabled or something elsewhere. I'll work it out; sorry for the misinformation.
You can enable it in the control panel, under 'Organization & users'. It's listed as Google+ but appears alphabetically after all the other Google things.