These thirty unnamed people lived in Monument, Colorado.
In 2008, while living in Monument, Colorado, thirty or so people from Monument, Colorado were possessed by the demons that were laying siege to the local sheriff's station. During this time, a few were killed by Ruby along with their demons when she fought her way into the sheriff's station. After the Winchesters and Agent Victor Henriksen allowed the demons inside the station, they used a recorded exorcism over the station's PA system to force the demons from their vessels.
The townspeople that had not been killed by Ruby survived their possession and the exorcism and in the aftermath were left confused by how they ended up in the sheriff's station.