Binary search. Some logic. Some API stuff. Quite hacked together.

RobloxServerFindAutoJoin.js

// @badccvoid
var PLACE_ID = 539494517
var TARGET_PLAYERS = 7

var queue = [0]
function req(placeid, startindex, cb) {
	$.get('https://www.roblox.com/games/getgameinstancesjson?placeId='+placeid+'&startindex='+startindex, cb)
}
function finish(id) {
	console.log('found maximum', id)
	req(PLACE_ID, id, function(res){
		var collection = res.Collection
		var server = collection[0]
		if (server === undefined) return finish(id-10)
		var first_players = server.CurrentPlayers.length
		console.log('first server has', first_players)
		// This is the logic for actually getting correct server to join.
		// Currently ONLY joins server with TARGET_PLAYER amount.
		// If none exist (e.g. only target+1 or target-1) it will cycle
		// back and forth until there is one. This is maybe not intended.
		// Feel free to update. 
		if (first_players < TARGET_PLAYERS) return finish(id-1)
		else if (first_players > TARGET_PLAYERS) return finish(id+1)
		else eval(server.JoinScript);
	})
}
function processQueue(min, max) {
	var item = queue.shift()
	if (item === undefined) return;
	req(PLACE_ID, item, function(res){
		console.log('fetched', item)
		var length = res.Collection.length
		var next = length > 0 ? Math.round((max+item)*0.5) : Math.round((min+item)*0.5)
		if (length <= 0) max = item;
		else if (length >= 0) min = item;
		queue.push(next)
		if (item == next) {
			return finish(item-1)
		}
		processQueue(min, max)
	})
}
processQueue(0,5000)