Chris- both you and the owner would be fine creating the registration for Portfolio Manager- there is no requirement for who does it. I suggest, however, that you encourage the owner to be the one to do it. The Portfolio Manager is meant for long-term tracking of a building's energy use and they may wish to use this tool long-term. Someone on the engineering side of your team, however, will need to be able to access the building's Portfolio Manager registration because you will need to establish a target finder score for the project through the Portfolio Manager and it is typically the project mechanical engineer that does this.