In Photoshop I am trying to apply a hue/saturation to just one or a group of layers instead of the entire lot of layers using the "Create new fill or adjustment Layer" button. Is this possible, and if so how?
Click the third icon from the left, on the bottom of the Adjustment Layer panel (just to the left of the eyeball) to clip the adjustment layer to the one below:
"Clip" means it will affect only the layer immediately below.
You can also do this by hovering the mouse on the line between the two layers, hold down Alt or Option, and when you see the cursor change to a double circle with a tiny, left-pointing arrow, click.
To have an adjustment layer affect only the layers in a group, put the adjustment layer at the top of the group and change the group's blend mode to "Normal" (as the default is "Pass Through").