No, its not turn based diablo. Diablo is hack&slash game with story-comvat is main thing in diablo and difficulty scales with number of players in game. In divinity story and quests are much more important, its not about killing everything and then again and again with bigger difficulty. Also divinity is somehow balanced for 4 player party(or 2 with lone wolf). And becouse its balanced for 4 players and story is important if you would be able to join and leave game whenever you want it would 1)break story element-suddenly there would be totaly different one of the main hero. 2)it would break difficukty if one the players had different level.

Its just isnt diablo with turn based combat. If u want enjoy story with your friend-dont play without him.