No. However, registering helps you get informed of any last-minute changes or updates.
Yes you can.
Campfires cover many themes throughout the year. Topics range from how to grow specific plants, to handling algae and techniques to get richer colours.
The default location is The 2Hr Aquarist 'Secret Lab'. Check out the posts on each Campfire Gathering for exact timings and location.
There is nothing like being physically present. But if you are not able join in person, look out for the link to join via social media.