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.
The backend especially is not too demanding (thanks to using a compiled binary via Rust). The database demands probably scale, but postgres scaling is relatively well understood. I think right now the least scalable parts look like the frontend node and websocket stuff, but that can be improved. I'm not sure how I feel about Activity Pub protocol wise, it feels pretty chatty, so transit scalability might be something else to consider.
We are currently removing websocket and switching to http, so that should be much better soon. Load from the frontend and Rust backend are both pretty low for now.
I believe Dessalines or nutomic were saying on a Matrix room that Postgres was their worst performing component at the moment, and why they needed a vCPU upgrade on their server. Maybe some sort of managed off-site Postgres instance could be an option?