You will not be be stealing XP unless you one-shot the mob. It's generally accepted that when inside a delve, it's group effort and you help each other out. It's only really a problem now in the early areas because they're crowded with new players and mobs go down in no time, so it might feel a little unsatisfying. In later zones when population thins out, I'm actually happy to come across another player and work together - veteran areas usually don't see many questers.I see it's a lot of different opinions on this topic, guess it might be best to just solo, and only interfere it it seems like it's needed