I do no know whether there is a collective noun for a group of tugs. My suggestion if there is not might be a push or a pull! Some of Southampton tugs are berthed at the Port's Dock Head as shown here in August 2009. On that occasion the group consisted of the Svitzer Sarah, Svitzer Lyndhurst, Svitzer Sussex and the Adsteam Surrey. The image was captured from the deck of the Steam Ship (SS) Shieldhall.