Honestly just tell them to sign up on an instance that matches their interest (if you know one) or a general instance (for most people, likely "yours" will do, idk fedia.io), they can worry about a new server once they figure out why they'd need one. I started on .ml myself like 4y ago and as soon as db0 existed I switched, for instance, once I learned .ml were tankies (well, I still had to wait because it was either them, germans, or midwest back then lol, but ykwim.)