If you look in the disABILITIES FAQs thread near the top of this board, there is a section of information about GACs. There are also some links to past threads and other information in the disABILITIES FAQs that should be helpful - including some past threads about travel with someone with autism.
Besides the suggestions you already got, another possibility would be for the whole group to go early in the morning and then the family with the child with autism leave before lunch. It sounds like something you will need to play by ear and be flexible about.
Also, about large groups. From personal experience, I would suggest not trying to stay together. Trying to keep a large group together is a lot like herding cats - the cats don't 'herd' very well and it's hard and frustrating work for the herder. With a large group, there are always people who are tired and want to rest, need to go to the bathroom, see an item they just need to buy while the group is passing thru an area, etc.
It's better (at least from my experience) to have everyone go their own way for the most part and meet up for specific things - such as everyone gets a Fastpass for some specific ride. Meet up there and go on the ride. Another possibility is splitting into smaller groups and changing the members of the groups from time to time so everyone gets to spend time with everyone else (this may not be popular with the child with autism, who may need 'his' group to keep certain members.
Not staying all together also gives more to talk about when you do get together.
Also, keep in mind that a GAC may help with giving a more appropriate place for the person to wait while they are in lines, but if the park is generally busy, you still have to contend with all the people and crowding to get from place to place, crowds in lines for bathrooms, to eat, etc. The GAC won't help at all with those situations. Waiting in lines is just a small proportion of the time spent in the parks.
If you want to stay together as a group, I'd strongly suggest using Tour Guide Mike (a Theme Parks Board sponsor) or some other touring plan to help you know what areas are busy when so that you can avoid them. Even if you split into smaller groups, advice like that would be helpful for the group with the member who has autism.