code: func (c *Client) AddServersToServerGroup(identifier string, serverIds []string) (*ServerGroup, error) {
  	group := new(ServerGroup)
  	opts := new(serverGroupMemberOptions)
  	for _, id := range serverIds {
  		opts.Servers = append(opts.Servers, serverGroupMember{Server: id})
  	}
  	_, err := c.MakeApiRequest("POST", "/1.0/server_groups/"+identifier+"/add_servers", opts, &group)
  	if err != nil {
  		return nil, err
  	}
  	return group, nil
  }
docs: // AddServersToServerGroup adds servers to an existing server group.
  //
  // The identifier parameter specifies the destination group.
  //
  // The serverIds paramater specifies the identifiers of the servers you want to add.