First of all, 마중 means 'going out and meeting someone who comes for a visit'.
마중하다 means 'to go out and meet someone when they arrive for a visit'. And it is a one word verb.
마중(을) 나가다 is a phrase which means 'to go out to do 마중'.
마중(을) 하러 가다 also means 'to go to do 마중.
As you can see the object marker '을' is dropped in 마중 나가다 and 마중 하러 가다.