Margrid

As I understood from what wiki writes I can only get her after beating the game. I know the place of dasha vertibule and pogahn passage but the mission starter request margrid. I have 2 questions. What should I do after beating the game to get margrid? İf I enter a group with margrid can I do these missions earlier.

Once you beat the game, the Heroes that you did not choose are available with "!" over their heads. You just accept the quests and talk to the person who will give you the Heroes.

And yes you can join a group who has Margrid to do these missions.