You can add new allies from different ways:
– by tapping on a character in the game world and selecting "Add as Ally" in the vertical menu,
– by tapping a character name in the chat window and selecting "Add Ally" in the vertical menu,
– by going to Social > My Allies or Suggestions > Add Ally, and entering the character name,
– after clearing a dungeon, you are suggested to add the group members as Ally.
Usually the first two options are the most commonly used 🙂