Yes this can be a difficult situation. It’s not easy when students are learning job-related language and they don’t all work in the same field. Business English is a vast field and some authors talk about general business English and specific business English.
However, this doesn’t mean that students are not interested in the topics that they don’t work in. Some of them might attend meetings in different departments or be looking for a transfer / promotion. It’s important to remember that the same thing happens when teaching general English: not all topics or language points interest all students.
There are a variety of ways to deal with this type of situation. Here are some ideas; you might be able to think of some more yourself:
Split each lesson into time for general business English or language study and time for specifics. In general business English, you could focus on using the telephone, language of meetings, etc. Many business course books are arranged around these themes.
In the specific topic part of the lesson, do a different topic each week so each person gets some coverage of their area of expertise. They also get the opportunity to be the expert in the group and to teach the others in their group about their subject. You could also ask higher level students to give a brief presentation of their work. Decide whether you will tell the students in advance which topics you will focus on. It could motivate some to attend; some might not come if the topic doesn’t interest them.
You could also do “topic clinics” for 20 minutes or so. For this, take in reading texts, vocabulary exercises, etc that are related to the fields your students work in. Students work individually or in small groups on the topic of their choice. Your role is that of facilitator. If you choose to do this, make sure the other part of the lesson is interactive. The topic clinic will cater to individual needs but will require you to do more preparation.
Finally, discuss your concerns with somebody at your school or in the training department of the company. You might find that they’re able to re-organise classes differently. Even if it’s not possible, you’ll show that you’re aware of the situation and that you are thinking of your students.
I hope this has answered your question. I’m not sure whether you were asking about how to handle the group or whether you wanted specific ideas for materials. If you want ideas about materials, please write in again.