Like Waters said, this is a horse riding metaphor. To harness means to take hold of; this requires sustained effort and control. To harness "their efforts" would be to direct these efforts along a certain path, towards a desired goal. It's less about working hard, and more about working "smart"; harnessing is just about control, not extra effort.

It is a horse riding metaphor: harness means to tie a reign to a horse. You would have to show the whole sentence, but generally it means 'trying very hard', or perhaps 'working hard together'.

I agree with Aaron, that "harnessing their efforts" is usually used in a context to suggest that someone can help focus and direct efforts so that people can work "smarter, not harder" all together. I think of the horses as being harnessed together to be able to pull a much heavier load all together, and to make sure they are all "pulling" in the same direction.