I work in an American subsidiary of a Japanese company. I met a few Japanese colleagues while working at the home office on an exchange. Do I need to use Keigo for:
Colleague A: 1 year younger in age than me and joined the company at a later time. It seems like I could use the desu/masu form without Keigo, but since we belong to different departments, different branches, etc., I'm not sure whether he is in-group or out-group.
Colleague B: Not 100% certain about age and seniority, but probably the same age and seniority as me or my junior.
Colleague C: 6 years younger than me and much less seniority. Should I use plain forms to talk down to him? I decided not to since I wasn't sure whether we were all in-group or considered out-group.