Even in cycling there is a lingo. When riding in a group its important and courteous to let the r person behind you know what you are doing to avoid a possible accident.
1. When wanting to stop, put one arm straight down to your side, palm facing towards the cyclists behind you to let the person know you are stopping (if you can not let go of the handlebar, simply shout "stopping") and make sure you keep a bit of a distance from the person ahead of you.
2. Car Up, say car up if you are riding in the street and there is an oncoming car approaching from the front.
3. Car back,say car back when there is a car coming from behind on the street. It is best to not to cycle side by side and go into a single file. Make sure you don't get too close to the parked cars on the side. (More next time on How to Ride in the Streets safely)
4. When making turns, make sure you put your right arm straight out to make a right turn, you can also make an "L" shape with your left arm (but rarely used any more). To make a left turn, put your left arm straight out.
5. If there is a car approaching from the left or right, let the person behind you know, by saying "car to your right" or "car to your left"
6. If there is an obstacle on the road, debris or something hazardous that you see and will be avoiding, let the person behind you know, by pointing to the ground
7. If you are coming to a stop sign (and you stopped, naturally) and you looked left and right and it was clear of cars, let the person behind you know by saying "clear". HOWEVER! My advice for the person behind is to make sure yourself as well that it is clear to go and not rely on the person ahead of you and ride blindly.
So there you have it. Become aware of your surroundings and you will be safe!