For this post, I took an explanation from an ICE developer, and added some illustrations and bit of my own explanatory text…
When you reference Group.PointPosition, ICE resolves the graph assuming the Get Data node outputs an array of 3DVectors in the context of the first primitive in the group. So, in this example, TRex is the first member of the group, so the context of Group.PointPosition is Array of 3D Vector per point of Trex.Trex:
In this case, the Array of 3D Vector is an array of four elements (the number of meshes in the group).
The indexset used (per point of Trex.Trex in this case) is reduced to its smallest size (the number of 0D from the geometry in the group with the least amount of points). The cube has 8 points, so you end up with 8 points for each group member. Since there are four groups members, you get 32 total points.