you can update the footage and pngs, as long as they have the same name as the pieces they are replacing they will update in any projects/comps that are using them (if the project in already open you will need to select the footage/image that changed and choose file>reload footage).
as far as groups working on the same project, my guess is that the project gets broken up into small pieces with individuals working on each separate piece. they are probably routinely outputting footage that is reassembled and overseen by a project leader… this master comp/project file could be available to the individuals too to see how their pieces are fitting in by importing it into their project and just reloading the footage in the master comp as they go…
when we work on large projects, like a station re-brand or news/sports packaging, there are usually pretty clear lines to divide up the project once the design direction has been chosen.
Kevin Camp
Senior Designer
KCPQ, KMYQ & KRCW