All NPC's, whether they work in the faction and guild commands, or as part of a plot-line, have certain duties and responsibilities, both to their faction, guild or plot and to the Lorien Trust. Players who reach rank 5 or 8 in their guild or faction are not automatically offered NPC positions; there are a limited number of these jobs available and they usually require a great deal of commitment and time, both in and out of character. Because of this, all decisions on the appointment of NPC's are made by the Lorien Trust on an out of character basis.
Promotion to an NPC rank is not part of a natural progression; for the purposes of role-playing ambition, it is usually best to assume that the highest rank to which your character can aspire is rank 5 in a guild and rank 8 in a faction.