Bathroom wiring question
You didn't say how the wires are connected, and what is already in the junction box. I'm assuming there is already a switch in the junction box, and that the switch is connected to the black wirenut and the the red wire.

In all cases, the white wires all go to the same wire nut. However, your existing wirenut may not be larger enough for 4 wires. You would have to know the color of the wire nut and the gauge of the wires, then look up a wire nut table.

If you want the light fixture to be switched, then you connect the black wire from the light to the red wire. If you want the light to be on all of the time, then you connect the black wire of the light to the black wire nut. Again, make sure your wire nut is not too small.