Hm. I'm not sure what they mean by "joining" their poly relationship, but that seems awful fast to me.
Building a relationship takes time. You meet each other. You date. You learn about each other. And then you decide if you're compatible and want to have a relationship together. IMO, anyway.
How can you know if you want to build a relationship with someone from an ad, a few photos, and few emails.
Yeah, I would be very cautious going into this. If they said they were interested in meeting someone to *potentially* have a relationship with, but to just .. jump in? I'd take it a step at a time.