If you want to set up a Fediverse group (a.k.a., "Public Forum") on Friendica, follow these steps:
1. Create an account on Friendica. I prefer doing this on https://venera.social/
2. Go to Settings > Advanced account types
3. Select "Community Forum" (a.k.a., group). Make sure Public forum is selected.
4. Click "Save settings".
If you want to see what a Fediverse group looks like, visit here: https://venera.social/profile/fediversenews
Also see screenshot.