Here are a few easy signals that we use so that we can communicate while riding. There's no brain surgery to this, but they will help.

To indicate to the rider behind you that you want him to pass or ride up next to you in the same lane -
Run your arm back and forth alongside your bike on the side you want them to pass on.

To indicate you are moving to the other half of your lane -
Stick your foot out in the direction you are moving

To let riders behind you know about a hazard in the road -
Point at it with your foot or hand

To let the rest of the group know that you spotted a cop -
Pat the top of your helmet with your left hand. Be sure to make this action noticeable to riders in the back of the group.

To tell another rider that they left their turn signal on -
Hold your left hand up and repeatedly touch your thumb to your fingers.

To indicate that you need to get gas -
Just point at your gas tank.

To indicate that there is a hot chick near -
Repeatedly pop wheelies, sit on tank, do a rolling smoke-out, or just do whatever your most impressive trick is