How understanding your audience will stop them attacking you

I was contacted by a PR company who was representing a psychologist. The psychologist had developed a workshop for the clients of a financial services firm. They asked me to facilitate that workshop.
The workshop was only two hours long and straight forward. It was about planning your use of time once you had retired. Even though I had many conversations with all the groups above, I didn’t investigate who was coming to the workshop.
On the evening of the workshop we were halfway through and I asked whether the workshop was making sense? One half of the room said the workshop was fantastic and the material really opened their eyes and made them think.
The other half of the room almost erupted in protest. “None of this makes sense”, “Only came here for the coffee” they yelled. What was I going to do?
As it turned out there was nothing I could do at the time. The group who were “up in arms” had already retired, so telling them how to plan for their retirement was not very useful. The other group who were full of compliments were years off retiring so found the techniques relevant.
If I had my time over, I would have rung the financial advisers and found out more about who they were inviting. They could then have invited more of the right people!