The team owns the team charter which is different from a project charter. Using scrum, the scrum master is the team's facilitator so they are an appropriate role to manage the artifact itself. That is the role after all which is responsible to ensure the team follows their own working agreements.
Team members are less likely to follow the working together agreements when they did not have a say in them though, so you need to make it *their* charter. Since the scrum master is responsible for ensuring the team follows their own rules, if the team is not doing so, they should figure out why. Does the team think they need to change their working agreements to be more effective, or are they not adhering to what they put in their own charter for some other reason? Saving Changes...
