I gather that you are in charge of the project and so you are senior to these people. If so, here are things that I might casually ask about over coffee, fwiw:

--What was one of the best projects you worked on in the past, and why?
--Who did you really enjoy working with in the past, and what was good about them?
--What area(s) might you want to work on next?
--What did you like best about our project?

If you felt comfortable sharing with them on any of these topics, I imagine that they would be interested.

From there, a natural bridge to slightly more personal topics might be:

--what films/shows have you seen lately and how did you like them?
--what actors do you most admire?
--what are some of your favorite movies/shows ever?

Would you be prepared to share with them about these topics as well? I imagine that would be of interest.

I propose these sorts of topics because of my imagining you being senior to them and not knowing them super well already. Many might be put off by a senior person asking about something not somehow connected to work. These topics are work-related afaik and so make sense to me in the setting you describe.