Oh, okay, there are a few groups.
The one that I have been to is Open Love NY. This group is growing, I hear (I haven't been in a while). They have a monthly cocktail party, on the second Monday of each month, downtown on the lower east side. I just posted the latest announcement here (I am hoping to go to this one):
NYC: Poly Cocktails announcement. I do like this group.
Open Love also holds discussions and I'm pretty excited about the next one. Betty Dodson will be speaking on Nov. 22nd!!! That will be at Manhattan Theater Club, on West 43rd Street (admission: $10 for non-members, $8 for members with bracelet).
Your FIL & his wife can sign up to receive announcements from Open Love NY by starting here:
http://www.openloveny.com/
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There's also another group that's been around a few years longer and meets every second Sunday of each month. I believe on the West Side. I have not been to any of those (yet). They have a Meetup page:
http://www.meetup.com/New-York-Poly-Meetup-Group/
And, there is The New York Polyamorous Women's Group, which also has a Meetup:
http://www.meetup.com/polywomenyc/.