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've got the same proxy config as in the NodeBB docs, but when I goto the URL I get a 502 page and errors like this in the error log:
[error] 23023#0: *698 connect() failed (111: Connection refused) while connecting to upstream, client: 18.104.22.168, server: community.ag-aus.org, request: "GET /socket.io/1/?t=1$2014/04/06 01:31:09