Just keep sending them cool memes from here or from Mastodon. Some will find them interesting and join up. Most won’t. Also, you’ll need some interesting group they will want to be a member of on Signal or Matrix or XMPP. That way, they also have a chat platform available for their friends as well.

And if you link to something in the Forumverse, link them through an instance you’d like them to use. So, even if the community is on some Lemmy instance, link to it through something such as piefed.social, if that’s what you think would benefit them the most as a choice for a home instance!
It’s a pity this does not work on Mastodon, though, because Mastodon always tries to push users to the instance where the message was written from, if the user is not logged in. Complicates things unnecessarily when the domain is different in every damn link! But with a widely federated Forumverse instance this problem doesn’t exist, because you can link to everything through that. And the people opening the link don’t need to understand anything about Federation. They just see this thing called piefed.social, and that there’s plenty of content you keep throwing at them. Without telling them to join.