Thanks @egomm. I have spent several hours searching the knowledge center but this post never came up. That said, I'm afraid it doesn't work. It does turn the points into multipoints, and will write all of them to a multipoint feature class, but it now ignores the points that originally and still have multipoint geometry.
Turn your points into multipoints, see here
https://knowledge.safe.com/questions/47959/convert-one-point-into-multipoint.html