If you have not read the GAC information in the disABILITIES FAQs thread, post #6 does have more information about GACs.
The usual number for a GAC is 5 plus the person with a disability (so a total of 6). The reason for this is often space because it is sometimes limited; there are some requirements in the ADA guidelines (not the law) that state there must be allowance for at least one companion along with the person with a disability. They will sometimes allow more than 6 on the GAC, but this is an unusual situation for special situations. There will still be need sometimes to split up because of space or sometimes the waiting area only allows for 6 to board there.
This is an example of a quieter waiting space (at the Laugh Floor in MK):
Many of them are for shows and are a roped or chained area next to the 'regular' line so that there is some separation between the guests. These areas are also used for guests with wheelchairs and
ECVs, so you will probably not be alone in the area, but will have more space than in the 'regular' line.
In some cases, like Small World
there is a separated wheelchair access, which can also be used by people with other disabilities. This picture shows the handicapped line on the right side of the picture, which can be busy and we have waited there for more than 1/2 hour when the regular standby line is less than 5 minute wait. The line to the left (that is full of people) is the exit line.
We have no choice, but to use it since DD is in a wheelchair. But for someone without a wheelchair, I would suggest looking at the regular line and using that if possible.
As was already mentioned, access/availability of quieter waiting areas varies from attraction to attraction. And, what you are told to do for access may vary from time to time, even on the same attraction on the same day. Most attractions do not have a way for guests waiting in a special area to meet up with the rest of their party when they reach the front of the line. So, if you are asked to split up, be prepared with who is going where and where you will meet up afterwards.
Many people have posted that using a tour service like TourGuide Mike or Ridemax has been more helpful than a GAC. The tour services point you toward the quietest part of the the quietest park. That can be very helpful because the GAC only helps you at attractions, you still need to contend with lines and crowds in other areas.