Be the Host. And I mean really be the host.
No matter how nervous you are, you have to take control. And the key to being in control is preparation.
As the host, you must welcome each and every person with a handshake and decent eye contact. Make people feel welcomed and at ease by explaining to first timers what the format of the event is and introduce them to others to start the ball rolling.
And most importantly - have fun. If you can have fun yourself, everyone around you will feel relaxed and enjoy themselves.