To want to join, I don’t know, tenforward.social and chat with other Trekies (and connect to other communities as a bonus) rather than to join “Mastodon”.
Language and countries instances can also help build a feeling of community
If they want to make an instance grow, and are ready to subscribe to most of the communities themselves, then sure, go for it!
If they just want something that works, the two suggestions above are quite good. Sopuli and DO aren't even that big now, so they can also benefit from the promotion
there is a niche community that doesn’t exist that you miss from reddit, I challenge you to make it here.
Please come to !fedigrow@lemm.ee for some advice. Quite a few communities get created then get abandoned a few weeks after, that's counterproductive as well
https://joinmobilizon.org/en/