This site is currently struggling to handle the amount of new users. I have already upgraded the server, but it will go down regardless if half of Reddit tries to join.
However Lemmy is federated software, meaning you can interact seamlessly with communities on other instances like beehaw.org or lemmy.one. The documentation explains in more detail how this works. Use the instance list to find one where you can register. Then use the Community Browser to find interesting communities. Paste the community url into the search field to follow it.
You can help other Reddit refugees by inviting them to the same Lemmy instance where you joined. This way we can spread the load across many different servers. And users with similar interests will end up together on the same instances. Others on the same instance can also automatically see posts from all the communities that you follow.
Edit: If you moderate a large subreddit, do not link your users directly to lemmy.ml in your announcements. That way the server will only go down sooner.
Following a decentralized identifier would essentially be like following a unique public key. Their screen name is just some text which can be anything. The bigger problem is the overall infrastructure of Fedi which is very much based on lists of user@domain .. this no longer trivially works if the "user" you follow could be posting from anywhere. It doesn't seem unsolvable, just kind of difficult to imagine with the momentum behind Fedi as it is right now.