As far as I recall, there were a limited number of times the organization was mentioned:
! <PERSON> shows that he is beginning to care more for the star. They are rescued by a passing sky-ship, on which <PERSON>'s hand is treated and bandaged. The captain of the ship reveals that he is a member of an organization which is later referred to as the "Fellowship of the Castle", whose agents have been working to ensure <PERSON>'s safe return to Wall. The man who gave <PERSON> the candle was also a member of this fellowship. The ship sets them down on the road to Wall
Source:
!http://www.thefullwiki.org/Tristran_Thorn
And the latter mention:
! After <PERSON>'s death, there were those who claimed that he was a members of the Fellowship of the Castle, and was instrumental in breaking the power of the Unseelie Court. But the truth of that, as so much else, died with him and has never been established either one way nor another.
Source:
!http://books.google.com/books?id=NQ_cSYevzAEC&pg=PA247&lpg=PA247&dq=%22Fellowship+of+the+Castle%22&source=bl&ots=j8n3ux1ZNH&sig=5H65dU3JeQKDzhVkjfj2eG4inhk&hl=en&sa=X&ei=ZDDOUJj6CYzq0QHX2YH4Dg&ved=0CF8Q6AEwCA#v=onepage&q=%22Fellowship%20of%20the%20Castle%22&f=false
The Fellowship of the Castle was also mentioned in the tale of Sir <PERSON>, which might offer a clue behind the meaning in the <PERSON> novel. However, it too was seemingly only mentioned in passing, making it difficult to determine its meaning. 773
Sources are also in old English and are difficult to interpret.
Sir <PERSON> waxed so faint of fighting and travailing, and was so weary of his great deeds, that he might not lift up his arms for to give one stroke, so that he weened never to have borne arms; and then they all took and led him away into a forest, and there made him to alight and to rest , him. And then all the fellowship of the castle were overcome for the default of him. Then, they said all unto Sir <PERSON>: Blessed be God that ye be now of our fellowship, for we shall hold you in our prison; and so they left him with few words. And then Sir <PERSON> made great sorrow, For never or now was I never at tourna­ment nor jousts but I had the best, and now I am shamed; and then he said: Now I am sure that I am more sinfuller than ever I was.
Source: http://www.kellscraft.com/KingArthur/KingArthurCh06.html
At last it seemed to him that the black Knights nearest the castle fared the worst, so, as he ever took the part of the weaker, he rode to their help and smote many of the white Knights to the earth and did marvellous deeds of arms. But always the white Knights held round Sir <PERSON> to tire him out. And as no man may endure for ever, in the end Sir <PERSON> waxed so faint of fighting that his arms would not lift themselves to deal a stroke; then they took him, and led him away into the forest and made him alight from his horse and rest, and when he was taken the fellowship of the castle were overcome for want of him. 'Never ere now was I at tournament or jousts but I had the best,' moaned Sir <PERSON> to himself, as soon as the Knights had left him and he was alone. 'But now am I shamed, and I am persuaded that I am more sinful than ever I was.' Sorrowfully he rode on till he passed a chapel, where stood a nun, who called to him and asked him his name and what he was seeking.
Source: http://www.sacred-texts.com/neu/trt/trt17.htm 381