jointhefediverse.net seems to be a commonly linked resource for directing people to join the Fediverse.
Curiously, it does not list Lemmy under the list of Reddit alternatives. Their GitHub README explains why.
Previous relevant discussion: https://lemmy.ml/post/78808
I see it happening gradually.
There might be a start with another Piefed instance (e.g. Piefed.zip, managed by Lemmy.zip admins). People who really don’t want to use Lemmy would register on that instance, but would still be able to interact with the communities on Lemmy, the way Mbin and Piefed alreay do now. They start hosting a few communities onn Piefed.zip, locking other on lemmy.zip and redirecting people there.
Then over time some other admins want to give it a try. After a while a few Piefed instances make it to the top 10 most active instances, while the rest is Lemmy.
It doesn’t have to happen overnight. We have time, people are not going anywhere.