I am guessing that he is trying to enter seige as it is ending.
I suppose that's possible. There are four stages to either Daily Siege or Organized Guild War, Registration --> Preparation --> Start --> End. Each lasts 1 hour and, as I've come to understand it, the End hour and Registration hour are both open to guild recruitment. It's still unclear to me if the 1 hour "cooldown" period remains for leaving one guild and joining another guild. Even so, if you leave your guild immediately after Siege or Guild War, you still have approximately 1 hour in which to join a different guild. I think YomanRoM figured that one out.
One thing for the OP to remember is that there's a tab (second from the right) in the Battles interface that shows the progression of Reg, Prep, Start, and End. The *total* time progression from Registration to Siege starting is two hours (Reg and Prep), so he'll have to pay attention towards the end of the Prep time in order to get into Siege. The Battles display has a countdown timer, in minutes, so it should be easy to figure out.