@FfaerieOxide Tell them to join the Fediverse from the Fediverse 😀
Like, simply, if you tell them that they can access all the content they want on Threads (because that's why they're there), while not being there, and also see it demoed from you, the person already on the non-Meta Fediverse, they are more likely to make the switch, especially if they encounter any regular issue like being wrongly banned or having network issues.
Hell, even if they create backup accounts here, it's still a partial win for us - they can switch back any time back and forth and see what fits them better. ;)