In 8 ball pool, what is the penalty if I play my group ball and another one of my group balls falls into the pocket without being touched?
The rules as defined at UPA are:
11.5 Hanging Ball If an object ball hangs in a pocket and drops in 5 seconds or less after coming to complete rest by the hole, the ball is considered to be pocketed. If a hanging ball drops in the pocket after being at rest for more than 5 seconds, the ball is returned to the original position on the edge, and the incoming player may begin his/her inning. Both players will have the opportunity to argue their case. The referee’s decision is final.
11.6 Suspended Balls If one or more balls become suspended in a pocket beyond the edge of the slate because it is partially supported by other pocketed balls, it is considered pocketed if the removal of the supporting ball(s) would cause the supported/suspended ball(s) to fall into the pocket. Tournament officials are the sole judges of whether this rule applies to any situation.
11.7 Settling Into Place A ball may settle slightly after it appears to have stopped, possibly due to slight imperfections in the cloth or table slate. Unless this causes a ball to fall into a pocket, it is considered a normal hazard of play and will not be moved back. If a ball falls into a pocket as a result of such settling, it is replaced as close as possible to its original position on the lip of the pocket. If a ball falls into a pocket during or just prior to a shot and it has an effect on the shot, the referee will restore the ball to its original position and the shot will be replayed. Players are not penalized for shooting while a ball is settling.
So no foul is committed.