Jen,
What you are wanting to do is to start an association which is usually done as an incorporated body.
This protects the individuals within that association from liability
You'll need at least 5 members
Here is a primer - lots of info on that site regarding the requirements
http://www.consumer.vic.gov.au/clubs...ub-incorporate
It would be good if another astronomy association could do you the favour of sharing their documentation, it might also be useful if you became a satellite of a major association - independent or not _ I am not sure, you would need to speak with them.
But it may bring significant benefits that you guys on your own could not - on the other hand it may limit your independence if you want to do your own thing that is outside the norm.
Genrally you want a public liabilty insurance policy to cover your activites - so that when someone gets crushed by a 20" telescope falling on them - they are covered - its usually a requirement of most council land use.
This association process is probably no different to the Pony Club or most other associations in the area.
If you dont incorporate you or another member could become personally liable, plus having a group of people who legally need to constitute a committee means that eventually others will share the load and hopefully it will grow a life of its own, that is more divers and better than any one person could achieve.
Good luck with it and hopefully someone from another association will share their paperwork with you, which you can amend and adapt for your needs without having to reinvent the wheel.
Rally