You should be able to find lots of oppurtunities for pairs work. Job interviews, yearly employee evaluations (good and bad) ordering supplies by phone, etc, should cover most of what you are looking for. You can demo a good and a bad example of each.
Also, assign each a topic (related to what they do in their office, ie, finance, materials management, HR) for presentation in class. 10 minutes each should be long enough. Give the topic and assigment in the first class so they have a few days to work on it. You could even have a practice year end business meeting in which each person needs to report on their division.

With such a short series of classes, you won't be able to help much with a lot of grammar or vocab, but maybe with giving the presentations and mannerisms. You can however give each of them a one page glossary of vocab related to what that specific person does, and they can study and learn from that in their own time. Having a small group like that affords you to be able to do that.