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.
Trolls impersonating the Lemmy developers has happened in the past. best is to report this to the instance admins who can delete the accounts as from the post history it is usually clear who the imposter is. Not sure if there can be a better way to handle this, probably not?
As @RagingNerdoholic@lemmy.ml says, create a distributed index of usernames, and do not allow the same username to be registered twice.
I’d also propose at the same time to create a Discord style username system to avoid potential clashes - if this system is going to become large (mainstream) then eventually available usernames will be hard to choose from.