Easy-peasy: In the Layers palette, copy a layer mask by holding Alt+Ctrl/Cmd, clicking the desired layer mask and dragging it to the destination layer. Move a layer mask by simply dragging it off the present layer onto the desired layer in the Layers palette.

Bonus tip: To invert the layer mask (to darken a background instead of lighting up a foreground subject), select the layer mask and hit Ctrl/Cmd+I.

You learn something new every day!

Move layer masks by dragging; copy by holding Alt+Ctrl/Cmd and dragging

Move layer masks by dragging; copy by holding Alt+Ctrl/Cmd and dragging