You can use Ajax to request their user page, and in the json it should specify all of the groups they're in.
ah sorry forgot to reply here,
meantime I found out, that its not a problem in custom JS to use app.user.groupTitle 🙂
it works fine so good
I would like to add a button in every post that before clicking on it will be gray and after clicking on it will be green for all users !!
This means that as soon as one user clicks on it he will color for everyone.
And that I can know who pressured him ...
Is it possible to insert such an element with the user ID that clicked on it?