There is a new rule that requires receptacles in bathrooms be on dedicated circuits, in one of two arrangements (pick one):
- receptacles in a bathroom are on a dedicated 20A circuit that only serves receptacles in bathrooms.
- receptacles in a bathroom are on a dedicated 20A circuit that only serves any load (lights, fan etc.) in that same bathroom.
Your setup is grandfathered since it predates this rule. However, you would break the grandfathering if you do too much work.
As such, if you have an existing receptacle in the bathroom, I would extend off its junction box for another receptacle in the same bathroom. If this circuit does not power any bathroom receptacles and your new receptacle won't be in the bathroom, then you can put "hot" with the existing blacks and "neutral" with the existing whites.
Otherwise you are not grandfathered and you need to pull a new circuit.
If you want to put another light off that same switch, it can either be powered off the existing lamp location, or this box. In this box, neutral goes to the neutral bundle, and switched-hot goes to the black-red wire nut.
Watch out for box cram/fill. If that's a 2-gang box, that's already pretty busy. Officially, count all wires except grounds as 1 each. Count all grounds together as 1. Count all cable clamps together as 1. Now count 2 for every switch or receptacle. Multiply by 2.0 if your wires are #14, or 2.25 if #12. You need that many cubic inches, and look inside the box for a stamped number stating cubic inches.
In your box I see 9 conductors + 1 (grounds) + 1 (cable clamps): 11. That is 22 cubic inches for #14 wire, or 24.75 cubic inches for #12.