I have a door that has a chain lock, like this:
Sometimes, my roomate locks me out with this thing. How can I be able to open it from the outside?
Image taken from the above site.
The one I believe in mostly is taking a piece of string and tying it to the chain, then threading it over to the outside of the door and pulling chain open. But I can vouch that all of these methods work.
For the rubber band method you can use a string that is tied, it doesn't have to be a rubber band. And if you are small you can stand on a stool and stick your hand in the door that is cracked and open the door that way. Wikihow also had a method, but it includes pushing against the door.