Ask if they want to go. That is your answer.

How's everyone getting along? My husband and I are friends with two couples with whom we vacationed. One couple, they don't want to be near each other but they understand that we're still friends with both of them. For our yearly ski vacation, basically one of them reached out, so she was the one who joined that year. The ex was understanding but said "you better take me next year." We also do frequent day/weekend beach house trips (family beach house) and essentially invite each every other time.

Our other good family friends split up and the wife can't handle me being civil to the ex husband (he's my dhs friend) and things got messy and we don't hang out with any of them.

So. Ideally you should reach out to the one you're closest to and say hey, what are your thoughts on the beach week this year? You're both invited but it's obviously up to you. Ball's in their court.

There are others in the group besides just you & divorced couple?

I saw reach out to the couple & say that you are planning for the beach vacation & up for whatever they want to do - if one parent wants to join with kids, if they want to skip, whatever. Hell maybe they are a unicorn couple & will show up together with the kids & coparent. Point is, it is in their court. Touch base with them & see what they are thinking.

Do they want to go?

For the last 11 years we have spent a week at the beach with a family we met though our DD's daycare. We became good friends and eventually rented a beach house as a group. It was the "Smith" and "Jones" family at first, but it morphed to other couples, families, singles etc.

Well, the other founding couple has split.

Do we carry on as if nothing happened? Does it matter their reason? This is all kind of weird but it seems odd to return to the same house and pretend nothing happened.