when I try to bring people on Lemmy, it’s always “why?” and if I make it that far, “how?” With the how, I’ve been using the analogy of signing up for email, though it’s still not as smooth as it could be.

You don’t have to bother with that. Just send them to a single instance to sign up. Stable, general-purpose instances like lemm.ee, sh.itjust.works, or lemmy.zip are good recommendations. You don’t really need to understand the details of how federation works to start using Lemmy. They can learn as they go.