Warbands are thematically grouped players either because of costuming, game organisation or roleplaying reasons. Warbands can consist of roleplayers and NRPs, as numbers will generally overcome the difficulty of a lack of character progression for Non-Roleplaying Players.
Warbands will usually fulfill NPC roles (roleplaying usually only occurs in Adventuring Parties). Members of the warband are all considered to be NRPs and will not be playing their own character(s). However, they still earn XP for performing as an NPC which can be spent on their character(s).