Aontaigh can mean "agree" or "unite", but "Do you unite with him?" would be an unusual construction in English. If anyone ever asks you An aontaíonn tú leis?, it's a safe bet that they are asking you "Do you agree with him?"
How would you say : I get on well with (someone)? I am trying to learn as many phrases and words as possible and thanks to all your (and other) comments my notes and my skills inrease a lot. Go raibh mílle maith agat