The highest forms possible! Use deferential and humble if you can. By the way, the masters won't expect you to be proficient in deferential Japanese, so if you just use the "masu" forms you will be perfectly OK. Just don't use plain forms, even if they use them towards you!

Don't forget to bow politely as well.

guys, thank you so much! Now I have pretty clear vision of the situation. Thanks a lot! :)

I think respectfulness for the visitor You can express respectfullness as your country way is best. We do not expect other country poeple perfectly use Japanese Keigo. I alway say to Japanese in other country. Roman do Roman's way (This is English we have same in Japanese). and Japanese are respects How you welcome that people that more important than words. even if your word incorrect they might be correct you word personally not in public but Just a few word how to say welcome poeple " Youkoso watakushi domo e Oide itadaki mashita" " we are appriciate you coming (Welcome)" " Wakakushi domo wa antagata no okoshi wo omachi shite orimashita (we look forward to see you) if you difficult Youkoso = welcome (polite way) Please remenber words not important, your heart is important. even if they are Our Emperor.

Since I'm not a native I won't answer the question, I'll just comment here. I would not use plain/short form. If you know keigo that might be the correct way to speak to them, but otherwise just basic polite form would be enough.