Hi. I am running a third light off an existing switch in my bathroom. They have a different configuration than I have seen before. Hot black wire comes into a switch. The white wire from the box is grouped together with all the other whites and never goes to the switch. I have added my new black wire from my new light to the grouping of black wires that comes off the switch. My white is connected to the group of other whites still never touching the switch. So my new light works great. But sadly all the time. The switch does not turn it off or on. What am I missing? It seems right to me but I have never had a switch like this. It is getting power but the switch doesn't control it. But the hot wire connecting to my light is coming off the switch. I am confused. Would appreciate any help.

You have connected the black wire to your new light to the hot black wires coming to the switch. Remove it and connect it to the other black wire that is connected to the switch.

That's because you added your new wire (at the switch) to the supply voltage side of the switch. That is power IN to the switch, NOT OUT.

Take it out of that bunch of wires (put them back together securely) and put it together with
The other wire on the switch, (called the switch leg, it is switched on/off power going OUT of the switch) and a short piece of wire to go under the screw with a wire nut.

This means you will have three wires in a wirenut, one OUT of the switch, one to the existing light, and one is your new wire.

1)you could have just put the new wire over at the old light fixture and tied it together with the same wires the old fixture uses.
2)switches don't need a neutral (white) wire unless they are lit or occupancy sensors.
3)sounds like you are just going by the color of the wires and expecting them to do the same thing every time, which is a mistake. Get a meter and maybe a book.