Adds a list of project members to a project. When the project is already assigned to all employees, it will be converted to a project with members. (assigned_to_all will be set to false) If the person is already a member of the project, the request is a no-op. If any of the person IDs are invalid, the entire request will fail with a 422 response. Subprojects can't have members. Trying to add members to a subproject will fail with a 422 response.