Grouping integers in Python

Question

I have been workling on a monte carlo collision method for gases in a latice. For this, I need to group some integers in an array. To be precise, I want to return an array, which groups the index of these integers. Here is what the program should do:

A = [100, 101, 233, 101, 243, 101, 233, 568, 233]

After grouping return:

G = [[0], [1, 3, 5], [2, 6, 8], [4], [7]]

While I have seen many proposals through "for-loops", I would however, like to accelerate this by vectorizing this process. Does anyone know a solution?