It has a directory server.

So basically it goes to that server and checks to see who is currently registered as a host. It gives list of each host with a Title (usually telling preferences I.E. 3D Fun Fly) and number of slots (users) supported and actual. Very simple very intuitive.

For example

Freds 3D FunFly 2 / 6 Users
Slope Soaring 1 / 2 Users
Open FunFly 5 / 5 Users

You can also connect directly to a host.

You can Host a session on the directory and just wait until folks fly in :-)

Or call a friend and have them directly connect privately (without registering on the directory server).

It's like going to a Chat Room. But you fly instead (and chat).

P.S. When I saw Multiplay I assumed it was the trainer cord thing and never looked further, I had no Idea it did this.