Translation:I'm having coffee with them at work.
'I'm drinking coffee with them in/at the office' should also be accepted, it is a more literal translation & makes sense in English
Normal English phrasings aside, isn't the construction "我和他们“ making the combined group the subject of the sentence? In which case it should be translated "They and I/I and they are having coffee at work." Translating "I'm having coffee with them at work" makes the other people into a modifier on the action, not active agents as part of the subject.
Not at work, but in the office or company. Please review. Shall flag this issue.