I don't know why but K keeps rejecting my invitation, saying "Very tempting but I've got the forest to watch over my Lady. I think you've got enough errand boys to satisfy your needs?"
Hi Sibella,
It sounds like you haven't spoken to Samuel in the Brookhaven yet.
When you arrive in Brookhaven:
Go to the Tavern and 'Gossip'. Water Mage will tell you to find the Great Seer's cousin, Samuel.
Go to Temple. The attendant won't tell where Sammy is unless you donate 50 gold.
Finf Samuel at the Fortune Shrine, bring 100 gold
Get Ruwin from Dungeon if you haven't already recruited him. report back to Samuel
He will tell you to lure Katnir into making a bet with you. Next time you spar with Katnir choose: "If I win, join my guild."
I believe this should meet all the conditions! Good luck!