It would be nice if you could sort by number of users/posts to see the most popular communities.
There was a recent discussion about this here: https://lemmy.ml/post/1151995, and among the few commenters there universal agreement we’d enjoy this kind of sorting and filtering options.
If you have trouble joining one of their communities, search for the link to the community, then your server will look it up. (search for the link, like this: https://beehaw.org/c/chat).
If that still doesn’t work, make sure that the server that the community is on isn’t banned by your server. You can check by clicking on Instances in the very bottom of the web UI.
kbin’s magazine browser lets you search for communities on kbin and lemmy as well, and you can sort by new, hot, and active