Good question. In the U.S. it is rare to see a group of monks; it is usually just one at a time, so then it is easy, you just do a simple bow with anjali.
For a group of monks, I'm not sure. Once my son and I were heading down Vulture Peak in India and we came to a large group of Thai monks. We weren't sure what to do, so just started bowing repeatedly.
Not sure if it was correct, because one of the monks asked (in perfect English) where we were from, reached out his hand to shake hands with me. I told him Las Vegas and his eyes lit up and wanted to hear all about it and how the Dhamma is progressing there, so we had a nice chat.