Finally found a way to the Gnoll Stronghold (yeah, not all exits from the map are the same). Swept all the gnolls out and acquired Dynaheir! And now I think our party is finally formed.
While I was searching the way in, I also explored a couple of new places and finished some small side quests (and changed sixth party member a couple of times too). Among them was Durlag's Tower, but I couldn't finish some riddles because I couldn't find the required potions, so I left it for now.