这里的 along 是副词，意思是“随同”“陪同”“一道”“一”。又如：

I took my camera along. 我带了照相机去。

She was willing to take him along. 她愿意带着他(一道)。

He took his dog along with him to work. 他带着狗与他一起上班。

Come to the party and bring some friends along. 请来参加聚会并带些朋友来

你的句子意思是：

He not only came but brought Mary along. 他不仅自己来了，而且还把玛丽带来了。