The origin of these signs is in sign language and different activist movements (for example the Occupy movement uses them too), but you don’t need to be an activist to use them, right?
This website offers a nice overview, including detailed descriptions of the signs, in case my drawing isn’t good enough for you :o)
I’ve made the experience that it is really important to introduce the signs and tell why it makes things easier. I usually just introduce a few, if people are curious and enthusiastic, I add more. I always draw them somewhere visible for the whole group too.
Well, that’s it from me. What’s your experience with hand signs?