code: func (s *ProjectsService) ListProjectCollaborators(ctx context.Context, id int64, opt *ListCollaboratorOptions) ([]*User, *Response, error) {
  	u := fmt.Sprintf("projects/%v/collaborators", id)
  	u, err := addOptions(u, opt)
  	if err != nil {
  		return nil, nil, err
  	}
  
  	req, err := s.client.NewRequest("GET", u, nil)
  	if err != nil {
  		return nil, nil, err
  	}
  
  	// TODO: remove custom Accept header when this API fully launches.
  	req.Header.Set("Accept", mediaTypeProjectsPreview)
  
  	var users []*User
  	resp, err := s.client.Do(ctx, req, &users)
  	if err != nil {
  		return nil, resp, err
  	}
  
  	return users, resp, nil
  }
docs: // ListProjectCollaborators lists the collaborators for an organization project. For a project,
  // the list of collaborators includes outside collaborators, organization members that are direct
  // collaborators, organization members with access through team memberships, organization members
  // with access through default organization permissions, and organization owners. You must be an
  // organization owner or a project admin to list collaborators.
  //
  // GitHub API docs: https://developer.github.com/v3/projects/collaborators/#list-collaborators