I think it's a great idea to get started. If you can find a group of parents who all are fairly sure their kids are heading to the new shool, then now's a great time to start.
What a leg-up if -- when the school opens -- you've got a couple of plans in place for welcoming/involvement events and you're already running. You may even be able to secure some budget from the groups that are feeding this new school.
The details (name, articles of incorporaton... that stuff) can follow.
Your organizing documents (Articles of Incorporation) have to show who your organization supports, so my guess is you'd have to hold off at least until there's a staff in place who you can talk to about forming a parent group, and whether the group will be a PTO or PTA.
