Old design morphing into new design

We are exploring options to move a portion of a legacy app into ES and ran into a situation I thought best to ask the community for help with. Again, legacy app.

This app has a rather heavy permission model with the peek user having about 1,000 assign groups they belong to. The documents indexed commonly have 1-6 groups assigned to them. There are about 30M documents and our peek user needs to filter on only the document they have access too. There are currently 120,000 groups in the system today and customers can add more if needed.

Crazy as it sounds, the legacy system in MySQL does a "select where group is in {1000 group id's}".

Suggested approaches?

Does anyone have an example of using Roaring Bitmap they can reference? I found an article but no examples to explore yet.