Project Online uses SharePoint Permission Mode, by default. This mode is more streamlined, enabling you to implement security quickly and consistently across SharePoint Online and Project Online. However, there may be situations where it makes more sense to use Project Permission Mode. For example, if you think your organization needs to manage access based on the RBS structure, you’ll need to use Project Permission Mode. This mode offers greater control, because you can customize and change individual permissions for each security group and typically intended for larger PMO organizations with a requirement to have more granular access control and has in-house expertise (or has engaged with a partner) to help with the implementation.
The worst is when you have a group and one person decides to "take charge" and "make a stand" and she's completely and obviously wrong. We all had our grades dragged down by one such on a group project we did, and the queer thing was she was a friend of ours who had always deferred to our higher grades and intelligence in the past. But on this one project she refused to go along, demanded to get her way "this once" on a subject where she clearly had no expertise, and the only way to get the project done was to either exclude her from the group or accept her ultimatum. We went along with her - she'd been a friend - and of course we got the lower grade. And to this day I still have no idea why she suddenly decided she had to have her way in this one thing where she was completely wrong.