The Project Team API consists of a set of methods that you can use to programmatically retrieve data about the project team and user custom fields.
You have to be authenticated to Jira in order for the methods of the Project Team API to be successful. Follow the Jira API documentation to see how you can authenticate to Jira. In addition, the user used to authenticate must have permissions for editing the custom fields of the other users. See Configuring the app permissions