For the moment, lemmy.ml is using an explicit allowlist for instances we federate with. If you currently run an instance, and would like to be added to join.lemmy.ml:
You can't see the remote communities unless you go to that site and browse their community list. But after anyone on your instance has subscribed to it (you do that by typing in [email protected] into your server's search box, and subscribing), then those posts will show up in All.
We decided to make Local the default tho, because once this grows, there will likely be lots of spam in All just like mastodon. And of course you can set your default view to Subscribed, Local, or All in your user settings.
Change your user settings to view All by default, or just click All on the front page to see federated posts.
Are there any plans to make it possible to browse a remote instance's community list without going to their site? That would make finding and subscribing to remote communities much more convenient IMO.