If I upgrade to premium, will all members of projects I own be able to use labels and premium features for those projects?
They can access any files that you add, but nothing else. They would need to upgrade to manage labels (they can still add labels) and add files to their own tasks.
I was shocked today to learn that a client I have shared a project with is unable to attach a file back to me because she is not premium. To be fair many of my clients will not pay for 'yet another task system' no matter how much pressure I put on them and I don't believe they should. Other collaboration programs provide premium features to non premium customers as long as the instigator is premium. Would this ever be considered?
I can't say it will "never" be considered, but it's not something I think will happen.