Both «Καλώς ήρθατε» and «Καλώς ορίσατε» are used to welcome someone. The first one is more applied to Modern Greek as a past form of the verb έρχομαι that means come. The second one is a more formal and literally type. It is the past form of the verb ορίζω, which among its others meanings, can also mean arrive.
So, basically the slight difference between these two is: Καλώς ήρθατε= well come and Καλώς ορίσατε=well arrived.
Of course, to welcome a friend you can say «Καλώς τον» (=welcome him) for a man or «Καλώς την» (=welcome her) for a woman.
The response there may be «Καλώς σε/σας βρήκα» (well I found you) .