What is a good way for a new Lemmy user to browse and join communities?
There are directories like https://lemmyverse.net/communities but it’s not straightforward for new users to simply click on a community to join
Are there any clients that have this onboarding workflow implemented well?
If you have the patience letting the drip of communities surface through your instances 'all' feed has been a good way to take in the growth.
I have an account I don't normally comment from I leave all the memes unblocked for killing time, otherwise it's been a lot of negative filtering and following interesting comments.
If you're self hosting that's a different problem.
If you use the voyager app they have an option to migrate subreddits using your multireddit. It basically parses the multireddit and allows you to quickly search for communities that match your old subs. It gives you directions to do it. I found it very helpful.