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'm currently working on a language community, in which there will be quite a few alphabet scripts, and was wondering if @ works with non-latin alphabets (e.g. @경은 will point to the user 경은)
@julian Close enough. Your point is made.