I am new to Symfony and am not sure how to best structure my web project. The solution must accommodate 3 use cases:
I would use two symfony applications. One for the public & members and then a separate for the admin functions. Unless the app is very complex and hard to maintain, it's simple to setup the security so that only members can view restricted content.
Assuming you're on a virtual dedicated or similar, setting up the subdomains is all done in Apache. If not, and you're using a shared host, then I believe it will be very difficult to setup symfony to work with subdomains.