code: func (client Client) UpdateOrganizationUserByUsername(orgGUID string, username string) (Warnings, error) {
  	requestBody := updateOrgUserByUsernameRequestBody{
  		Username: username,
  	}
  
  	body, err := json.Marshal(requestBody)
  	if err != nil {
  		return Warnings{}, err
  	}
  
  	request, err := client.newHTTPRequest(requestOptions{
  		RequestName: internal.PutOrganizationUserByUsernameRequest,
  		Body:        bytes.NewReader(body),
  		URIParams:   Params{"organization_guid": orgGUID},
  	})
  	if err != nil {
  		return Warnings{}, err
  	}
  
  	response := cloudcontroller.Response{}
  	err = client.connection.Make(request, &response)
  
  	return response.Warnings, err
  }
docs: // UpdateOrganizationUserByUsername makes the user with the given username a member of
  // the org.