The way to grow the fediverse is getting real life communities to build local instances and connect them up with others.
A good example is Universities - these are dynamic communities pf young people that could benefit from campus wide social networks of Mastodon, Lemmy, pixelfed etc. Joining these up with other university networks would create shared spaces, and as people graduate and leave university they would hopefully continue to participate in broader genera sharedl communities.
Other communities to target might be small towns, neighbourhoods or even cities.
This is exactly how Facebook grew. It target universities and green organically - although it did also use an idea of "exclusivity" and invites to draw people in to the network which may not be feasible for the fedoverse. But the "local social network" to "global social network" itself is a viable route to growth.
So instead of people trying to persuade individuals to join a global network they should be thinking about how they can support exisitng local or small groups onto things like a campus social network. Sports clubs, hobby groups, and even just popular shared hobbies and unterests. Things that people are already in communities for but may appreciate their own locally controlled instances of fediverse tools which they can federate into bigger whole.
Localism is a big strength for the fediverse and is where I'd target growth.