If you are talking about tour charter bus, you have many times getting on and off of one group.
For first time of getting on time, usual greeting words is enough, I think. Like,
"ohayou gozaimasu" (=Good morning)
"konnichiwa" (=Good afternoon)
"konbanwa" (=Good evening)
Also "Boston e youkoso!" (=Welcome to Boston!)
Then after above, after they return to your bus from some sightseeing, you can say
"okaeri nasai" (=Welcome back)
Anyway, if you make mistakes, they appreciate your Japanese words, become closer with you. Don't worry.