After some messing around with angles and what we are holding, we were able to glitch into the piano so now were are on the ground level of the building not the floor (we are literally standing within the piano). Once you achieve this effect you can freely move under the piano and into the walls but its not THAT easy.
1. You can not be "Double-carrying", have nothing in your hands or it wont work.
2. Even though to you it looks like your in the wall, to my friends client my player model would get hung against the piano/wall.
-I got to the point where I have reproduced this glitch with a friend many times that I know the pattern/path to take to make my player model disappear with out any guidance from third party. If you have done it enough you will just know.
Once you are inside the Piano Building just stand in the middle of the main floor, holster your weapon and run twards the angle in between the bench seat and the piano, depending on the geometry of the ground that the house is sitting on it may take a couple tries but if you have a flat ground outside and under the house it should take effect in the matter of seconds, just run in that corner just right and you will just pop into the piano and should be standing on the ground, not the floor of the building anymore. Now this is where a friend comes into play. You don't walk more into the left corner of the piano/wall, you need to sync your view with your player model that your friend can see, if your friend is standing in front of the "fake/static" door to the right of the piano and you just got into the piano, slightly move to the wall but not into it and slightly walk to the curve in the piano. Now your friend can guide you in. He will be looking at your player model, if he sees that your player model kind of just drop into the ground he will let you know and tell you to move under the piano. Now if your view is in sync with your player model that your friend sees, you can now just look at the wall and walk in, you and your player model should just seamlessly walk into the wall without your player model getting hung on the wall. I have noticed too if your in a rush and you have someone inside the wall already and your guiding a friend into the wall and his player model is hanging there but hes walking through walls, just get him to come to you if your in a different section of the wall, it will sometimes just sync for both you and your player model. Now that you are in the wall and an invisible wall is blocking you from progressing through the house just tap "A" and "D" as you move forward and you will shimmy though the invisible walls inside the walls and if your with friends/friend you can just get him to hold your secondary weapon while you run into the piano wall glitch quick, he will drop your weapon on the floor inside piano building and you just crouch to your weapon and pick it up, so now you have both weapons and your inside the glitch.