Yes, of course if you are addressing more than one person you have to say "Ustedes" or "vosotros".
In this case:
Iré a visitarles muy pronto, eso espero,......
Les visitaré muy pronto, eso espero,....
You don't need to say "ustedes" in the sentence since the verb is already conjugated in the right person, so the pronoun is understood. But you can if you want.
Regards.