GroupMining.js

function GroupMining() {
	this.member_ids = [];
	this.semaphore = 0;
	this.groupsCount = 1;
}
 
GroupMining.prototype = {
	unlock: function() {
		if (++(this.semaphore) === this.groupsCount) {
			resultStr = this.member_ids.join(',');
			console.log(resultStr);
		}
		return this.semaphore;
	}
};
 
FB.getLoginStatus(function(response) {
	gm = new GroupMining();
 
	if (response.status != 'connected')
		return;

	// ACM's Group ID is 228954137162541
	// Repeat this call for every group whose members you wish to mine
	FB.api('/228954137162541/members', function(result) {
		for (var i = 0; i < result.data.length; i++)
			gm.member_ids.push(result.data[i].id);
		gm.unlock();
	});
});

Needs a set data structure to prevent duplicate IDs from reappearing in multiple group queries.