Remember if you do:
Don't get caught up in the technical details, those aren't what matter to many people. Obvious exceptions if they express interest in hosting a server.
Don't talk in the jargon of fediverse this, federated that, those mean nothing to outsiders. Unless they ask for further details, but even then, use more familiar terms like saying it's a website connected to other sites or whatever might help bridge understanding.
Do point them to whichever instance you're on, or seems relevant to whatever interests they may have mentioned, to join. One of the big problems people mention is figuring out which site/instance to join and how to navigate around here, take this opportunity to help them cut through that.
Ask them which subreddits/accounts they've joined or like to follow where they are as relates to the platform they're trying to move from (i.e. Reddit/Twitter/etc.), and try to help point them to either their equivalents or similar that may be found around here.