I'm looking for something that can put one item into potentially multiple groups?
For example, given the following input:

[
  { name: "x", groups: ["a", "b"] },
  { name: "y", groups: ["a"] },
  { name: "z", groups: ["b"] }
]

It should produce the following output:

{
  a: [
    { name: "x", groups: ["a", "b"] },
    { name: "y", groups: ["a"] }
  ],
  b: [
    { name: "x", groups: ["a", "b"] },
    { name: "z", groups: ["b"] }
  ]
}

I'm wondering whether any programming languages have such a utility in their standard library, and if they do, what is the usual name?

Here's a possible JavaScript implementation in case the desired behavior isn't clear from the example:

const multiGroupBy = (items, callbackFn) =>
  Object.fromEntries(
    Object.entries(
      Object.groupBy(
        items.flatMap((item) => callbackFn(item).map((group) => [group, item])),
        ([group, _]) => group,
      ),
    ).map(([group, items]) => [group, items.map(([_, item]) => item)]),
  )