Thanks to everyone for your advice and stories so far! We've decided to start out with a quick intro and handshake before their next date (to break the ice, in case she is just nervous about meeting), and then move on to a longer discussion about our expectations, maybe at a coffee shop or over a cup of tea at our home. That way, if not wanting to meet is an indicator of any serious issues she may have with polyamory, we can have time to discuss it. If anyone is interested, I'll post an update to let you know how it goes.
Looking forward to reading more replies!