Well I’d just tell them if I was recommending it to a friend or making a post like “follow me to mastodon” or whatever, and someone curious enough to find mastodon without a recommendation oughta be able to figure out an instance to join, mastodon.social is the first result when searching “mastodon” so it’ll probably get them just based on that.
Tbf, some people want that. I prefer my instance which federates with anyone willing making you choose what to block yourself (aside from the reactionary instances that don’t like that we don’t block their enemy instances by default so they block ours, of course), but not just anyone can join my instance, so I can’t recommend that unless they qualify. If you have a better general instance I can recommend instead of .social I’ll definitely check it out!