Field trip groups, nature troops, enrichment co-ops, STEM leagues, book clubs, university model classes, PE & Park meetups--after fourteen years of homeschooling, I've participated and/or led every type of homeschool co-op imaginable.
The problem with homeschool co-ops
Some of these groups became like the bread and butter of our homeschool. These co-ops were indispensable to us. They offered engaging classes that filled in the holes of our homeschool. They were packed with friendly and generous people--families willing to invest in each other. And they gave us all an opportunity to learn subjects and skills that we'd otherwise never be drawn toward.