thats weird, i invited 8ish friends, 4 are on the same server (different league), 1 on another, 2 started on their own and the last piggy hasnt started yet
even though you invited them via the app it doesnt mean youll get a 100% chance of you guys being on the same server. who knows, you probably had terrible luck
either that or they probably started via someone elses invite, i smell jelly XD