I think the easiest way to start is to ask questions that you need for your own GTD and make it a consistent, visible vocabulary.
I suspect you are already doing it, if you are chairing the meetings, by asking for "the next actions" to complete the meeting.
I found that if I keep asking "So, what's the next action and on whom" over and over again over a period of time, people I work with are starting to understand the mental steps I expect them to go through.
I also keep Waiting For items on things delegated and make sure to follow up and mention explicitly that I will follow up again on specific date.
This allows people to have their own methods as long as they can provide you with actionable information. I disagree with @Renan that Autonomy is an excuse for inefficiency. I think most of the time, the inefficiency is just a lack of return on investment for doing things efficiently, such as broken feedback loops, lack of consequences for failure, inability to escalate, etc.
Of course, as a consultant, you may not have enough visibility and duration to affect real change. That's a true challenge.
(Addendum) And if somebody gets moderately interested, I bought a bunch of GTD classic books and "lend" them (not really expecting them back). At $10 a book (from Amazon), it is cheaper than spending lunch time explaining the process. And it gives me a chance to follow up with them later and ask whether it worked for them.