The trackers will usually meet you half way through your trek and some may go along with you once the gorilla group has found you, or you have found them. In our case, the gorillas were swinging on bamboo branches over the tops of our heads before we had any idea they were close. The trackers know the gorillas better than anyone, they stay and protect them long after you have left and they are the ones that radio in their location. They're off finding the gorillas while you are still enjoying your early morning cup of tea. The system works, it saves a little trekking time for the tourists and also pretty much guarantees you will find the mountain gorillas you have paid so much to see.
The porters are local villagers who will meet you at the various parking spots close to the start of the paths leading into the forests. Their rate is about $10. Even if you don't think you need an extra hand carrying your day pack, or help you navigate the ridiculously slippery terrain, it's worth employing these guys anyway. It's a simple way of helping local villagers benefit from gorilla tourism. The villages around these parts are very poor. If local men can earn a bit, it encourages them not to turn to poaching, or encroaching and planting within the park boundaries.
An armed ranger, employed by the park, goes along on every gorilla trek mostly to keep you safe from the elephants and buffalo that hang around the lower slopes of the volcanoes.
Bring enough US dollars to give everyone a tip at the end of the trek. You should tip the trackers at the end of your hour with the gorillas, since they do not head back down the mountain with you.