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.
You have to make a new account unfortunately.
Seems like this should be a high priority feature. I did try joining a different instance but at the time only lemmy.ml was functioning and accepting applications. Now that I have subscribed to a bunch of communities starting afresh would be a bit of a hassle.
It was apparently not a high priority back in 2020, and there hasn't been any movement on the feature request: https://github.com/LemmyNet/lemmy/issues/506
Well that's not a good sign. I hope the potential flood of users brings a flood of devs along with it.