There is no forum that I know of which is dedicated to travel buddies for mileage runs but there have been threads in airline forums bringing together people taking advantage of mistake fares and sale fares. Here are some examples.
The BA Viking meet-up thread (aka. ex Norway CW mistake fare)
The BA Bratwurst meet-up thread (aka. ex Germany F mistake fare)
The Satay Fare EDI-PEN-EDI
Not sure which airline or alliance is offering attractive US-India-Singapore fares you mention but it may be worth checking the relevant airline or alliance forum.