One of the homeschooling groups that I belong to formed a cooperative about a year and a half ago. My family does not participate in the cooperative because it is not the best fit for us. As a family, we would prefer to barter skills with another family. Exchanging knowledge in a more one on one basis feels comfortable and less stressful. It is also more flexible for us that way.
Co-ops, as they are more commonly known, come together because a group of parents got together and decided to volunteer their talents and experiences and "teach" a class. One parent may be strong in math and another may be fluent in a second language. Some older children can even teach a class. For example, if someone has a teenager who took dance lessons as an itty bitty, that teen may want to teach a group of 5 to 7 year olds some dance steps.
For those parents who are not hosting a learning activity, it is important to be present to be responsible for your own child(ren). Co-ops are usually not an experience where you can drop off your child(ren), run errands and pick them up in a couple of hours. Medical issues, behavior issues and meltdowns happen to the most angelic of offspring. It is also important to be available to provide any possible assistance.
When you choose to participate in a co-op, it is a commitment. Most co-ops operate in block sessions. For example, a co-op may run a series of classes for 6-8 weeks, break for a period and then start up again for another 6-8 weeks with new classes.
Most co-ops are free. Other may choose to charge a "tuition" fee or ask for a small fee to cover materials.
Cooperatives are not for everyone. Visit a co-op class before deciding to join. Get to know some of the members and understand the philosophy and/or expectation of everyone involved.