As Balaji's link, nGrinder has some features which facilitate cooperative performance testing.
Usually, the problem which Colin faces comes when people uses same account to access the nGrinder.
Instead.. I highly recommend you to provide each other's account per a developer.
Then.. use the user account share feature to develop and run the script together.
http://www.cubrid.org/wiki_ngrinder/entry/user-account-share