Very interested in the answer to this as well. I’ve never DM’d but am really thinking about it for my current group. We’re all on 5e though and I wouldn’t expect them to want to pony up for a completely different ruleset just to try it.
One of the strengths of D&D Beyond is how someone can purchase and share their library with others, and also how there is a subclass of each class that people can try for free to see if they even like it. Really hope Daggerheart/Demiplane is planning on either/both of these things.
I can answer the first question you asked. Unfortunately more information isn’t available just yet for the other questions you have. As soon as that information is available however we’re going to be as loud as possible to make sure it gets communicated to everyone who needs to know!
If I want to GM for my group, will they need to pay as well? If not, what exactly will they have access to (assuming I’ve paid)?
If you’re a subscriber you can share access to the content you’ve purchased with up to 24 players. This way they don’t need to purchase anything and they’ll have access the same way you do!