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.
I think this is the part I see as a potential hurdle for hosting a primarily auth-purposed instance. Fetching remotely would be too slow but data replication would exponentially increase storage over time. So anyone planning to self-host would need to take on the storage growth costs of all other federated instances raising the barrier for entry over time. Unless I've misunderstood this point
You only need to store the content from communities that you follow. So in a single user instance it will be very limited. And anyway its all text which doesnt take much space. Remote images/video are not mirrored.