code: func (s *IssuesService) AddAssignees(ctx context.Context, owner, repo string, number int, assignees []string) (*Issue, *Response, error) {
  	users := &struct {
  		Assignees []string `json:"assignees,omitempty"`
  	}{Assignees: assignees}
  	u := fmt.Sprintf("repos/%v/%v/issues/%v/assignees", owner, repo, number)
  	req, err := s.client.NewRequest("POST", u, users)
  	if err != nil {
  		return nil, nil, err
  	}
  
  	issue := &Issue{}
  	resp, err := s.client.Do(ctx, req, issue)
  	return issue, resp, err
  }
docs: // AddAssignees adds the provided GitHub users as assignees to the issue.
  //
  // GitHub API docs: https://developer.github.com/v3/issues/assignees/#add-assignees-to-an-issue