As far as people receiving a first time outside of Mass I can easily think of a couple of circumstances.
One might arrive while a Mass prior to one’s intended Mass was still going on and, for whatever reason, wish to receive from that particular celebrant.
Or one might be with a sick relative or friend when some comes to deliver communion to that person. If one has not yet received communion that day then it would be possible to receive communion.
In both of the cases mentioned above one could later receive at a Mass at which one participated.