code: func (s *GroupService) Add(groupname string, username string) (*Group, *Response, error) {
  	apiEndpoint := fmt.Sprintf("/rest/api/2/group/user?groupname=%s", groupname)
  	var user struct {
  		Name string `json:"name"`
  	}
  	user.Name = username
  	req, err := s.client.NewRequest("POST", apiEndpoint, &user)
  	if err != nil {
  		return nil, nil, err
  	}
  
  	responseGroup := new(Group)
  	resp, err := s.client.Do(req, responseGroup)
  	if err != nil {
  		jerr := NewJiraError(resp, err)
  		return nil, resp, jerr
  	}
  
  	return responseGroup, resp, nil
  }
docs: // Add adds user to group
  //
  // JIRA API docs: https://docs.atlassian.com/jira/REST/cloud/#api/2/group-addUserToGroup