Converting attic into games & bedroom
Joists can normally easily be sistered. This is when another usually larger one is nailed or screwed to its side. This stiffens a floor quite a bit. Depending on your existing joist, load and span would depend on what side you would go with. The draw back of this is your floor is raised above the original location making your ceiling height lower and may mess up your flow on the up stairs a bit (all should be considered). What ever you do, use the screws. Nails unless using a nail gun will for sure stress your lower ceiling causing damge.