If you want to communicate with another member but not necessarily in the forums or their public pages, you can send a private message. The feature is marked by a little envelope, but initially you will only see it next to the names of site administrators (me and a few other people). If you want to send a PM to someone else, you have to "friend them" first, which is to say you extend a friend invitation to them and they accept. You should then see an envelope icon next to their name at all times, and you can PM them at will.
Ning imposed that restriction last year to cut down on PM spam. It's kind of a pain, but you can probably see why it makes sense.