Silverstar is correct. You can click on the member directory, search for the member you want to follow (there's a sort drop-down list with various options), then, once you see their avatar click on the "follow" button. You can also follow someone that you see on a discussion by simply hovering over their avatar. When you do that, it will enlarge and there's a follow button at the bottom.
In order to send someone a private message (at the moment), both of you have to be following each other. Again, simply hover over their avatar and you'll see the enlargement with a little envelope on the bottom right side. Click on it to send a message. You could also follow them and do an @ mention of them to get their attention (assuming they have notifications turned on.) Once you've gotten their attention you can ask them to "follow" you, and then you'll be able to send messages.
