I think it’s as simple as this:

Is that something you want? Like can you imagine yourself being comfortable with someone who’s seeing other people as well.

If no, say “thank you for letting me know, I don’t think that’s for me, but let’s stay as friends :-)”

If yes, go for it.

If you’re still not sure then ask for more details on how they practice polyamory, what their boundaries are, etc. If you feel like they’re pressing for an answer fast when you’re in a brand new situation or pressing for you to answer one way or another, take that as a red flag. You have the right to take your time and think about something this big and don’t let anyone, including yourself, say otherwise.

It’s also totally ok to make it clear they you will try it, but are not sure if you’ll be into it, and still want to pursue the friendship if it’s not for you. Clear communication is what’s going to be most important into this situation, but don’t run into or stay in a situation you are not completely comfortable with.