search the internet (like Flickr) of various poses for multiple people. Then you have to try and incorporate that into what ever objects are on the roof. Also don't forget to check the background too.
I've seen some really good multiple business poses
Do you have strobes? What kind of gear do you have?
I think if you wait until a nice sunset, about an hour before the sun goes down and get a nice sky behind them with a lot of buildings, that could be cool.
Try looking on Pinterest (as much as I hate it, it works)