You can just grow it organically. Both schools our kids attend have Facebook pages for parents, and they grow on their own over time. Parents tell other parents, and in a year or two it grows more quickly. You can also IM the current members of the sight and discuss next steps. You could create a new one that is run by parents, and all switch as a group. lol
To me, the best about an “unofficial group” is that it is not overseen by the school, so you can discuss honest concerns with other real parents. If your kid has an issue, it is good to know whether it is just them or many others have the issue. It may impact how you deal with the problem.