I think that there are a few ways for you to 'invite.' If you want to invite someone already on Classroom2.0 as a 'colleague' or 'friend,' you should be able to do this by:
1) First go to a person's member page (like mine) and
2) select "Add as Colleague" (look on the left of the page, under the person's image
At that point, the Ning site will send a friend/colleague request to that person.
If you want to invite a friend who is not yet on Classroom2.0, but whose email you have, you can go to your own page, and on the right side of the page, you'll see a 'Colleague - Invite" link. Click on this, and you'll be able to invite friends who might not be on Classroom2.0 to the community.
The last way (that I know of) is to look at the bottom of a discussion thread or comments list, and select "Follow" at the bottom right of the thread.
I hope this helps --
I'm going to 'invite you' to be my colleague, too! =)