For example, my 12-year-old daughter is an avid reader. She has a very bright homeschooling friend who is having difficulty with reading. She's 11 years old. A third friend, 13 years old, can read but doesn't particularly enjoy reading. Now if we were to do a "reading workshop," we would run into a lot of difficulties and even duplicate many of the problems found in the public school system. So each parent works individually on each child's reading level.
At the thought of "studying history," my daughter glazes over and pretends to fall asleep. However, if we read an American Girl book as she's playing with her American Girl doll, she can't seem to get enough. We're currently doing a colonial theater group. My 12-year-old is helping my 7-year-old daughter and several of her friends put on a colonial play. The younger and the older kids are working well together.
I might add, there were several boys who were part of the original group. After the first session, they dropped out. Boys' needs and girls' needs often conflict, and if you are depending on a group learning process, you may be disappointed. The age differences you mention might also eventually become an issue if the children are grouped together for too many activities.
My suggestion would be to let each child discover her own learning style. Give each one some freedom to pursue interests. Incorporate basics each day for the older kids -- maybe math and spelling -- and get together as a group for special projects. Maybe a science club that meets once a week, with one child being responsible for a weekly experiment. Or maybe a writing club at another mom's house where one child starts a story, and each child adds a page until it is complete. I wouldn't force anyone to do anything -- that never works and breeds resentment.
One thing that will guarantee the success of your group is that you have each other for support. Those who have difficulty with homeschooling cite lack of support as a major issue. Be sure to network with other homeschoolers in your state. Find out what has worked for them, and participate in some of their activities. Read books on homeschooling, and start with The Homeschooling Book of Answers by Linda Dobson. The first year will bring surprises, but after that initial adjustment period, I think you'll be glad you made the change.