For some difficult group situations you may not be able to go in with a camera. For these cases you can film at a distance and send in someone wearing a wireless microphone.
On the Fly
If you are attempting to do spontaneous filming at a location you are going to encounter a number of problems. If your documentary is covering a controversial topic your recording presence may be not be completely welcomed. One technique you can use when trying to enter somewhere like this, such as a protest or rally, is to attach a wireless microphone on a person and then follow them with the camera at a distance.
This is not always going to get the best footage, but what you do end up filming can be of great use. What you will end up getting is very clean audio of what is happening in the middle of the crowd or group, while getting an external master shot in the visuals. From this you can still maintain continuity because the sound will be incredibly accurate. From this sound you can then cut around and try and get as many visuals to accompany it as you can.
To attach this microphone it is best to secure it under the clothes, keeping all elements of the device hidden. Though a camera may aggravate some people, the appearance of a hidden microphone may make them completely irate. Make sure to test out the microphone several times once it is attached to make sure that it will actually work once the event is to take place.
Make Them Easy to Find
Have the person dress is fairly bright and identifiable clothing so the camera person is able to follow their image from a distance. From here you will be able to keep the image focused on the where the audio is coming. This way the final image will match accurately.
Get In There Anyway!
No matter what, you should attempt to storm into the crowd as much as possible with the camera anyway. Save this until the end because you may have to leave immediately after you do so. Get as many shots as you can while using the wireless microphone, and then make your attempts into the crowd. The best way to do this is to notice where the most violent factions are and avoid them. Never go near police that are responding to a situation like this because they will be quicker than any group to attack a person filming them. Either way you are there to produce a documentary of importance so taking risks should be acceptable. But you still should use your personal judgement and caution.