Hola Kaly

In this case, the message is "stating" something that they "can indeed do", there's no uncertainty or doubt about it, hence the indicative. 

If we hadn't used the modal verb and just say "ask them to come in" instead, that would take the subjunctive: 

Dígales que pasen. 

This is when we use decir with that meaning of "asking someone to do something". 

In the original example "Diga a los señores Moreno y Ruiz que pueden pasar a mi despacho." what we're saying is "Tell them that they can come in" - this "that they can come in" is a true fact that the narrator is stating.

I hope this clarified it.

Saludos 

Inma