from FAQ: Will I be able to play with my friends if they have a Steam account and I don't?
Yes. The game and experience between the two different types of accounts will be exactly the same. Players who have an account registered through the website or through Steam will be able to play together as the game is identical and use the same servers. The only difference is how players login.
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ok, I'm just creating a new character. and start playing again with friends on Steam from Guild Wars 2 website Because I already bought the DLC in the system. it's ok, Let's start playing again and have fun with new friends

from FAQ: Will I be able to play with my friends if they have a Steam account and I don't? Yes. The game and experience between the two different types of accounts will be exactly the same. Players who have an account registered through the website or through Steam will be able to play together as the game is identical and use the same servers. The only difference is how players login. .................................................................................... ok, I'm just creating a new character. and start playing again with friends on Steam from Guild Wars 2 website Because I already bought the DLC in the system. it's ok, Let's start playing again and have fun with new friends