Let's put ourselves in the shoes of the client. Assume that each developer is billed out at $100 per hour. If you have a 1 hour meeting, and there were only 2 people present, then sure, $200 makes perfect sense (provided the meeting was constructive and there were demonstrable, value-adding outcomes). However if I'm your client and I receive a bill of $1000 for a 1 hour meeting because there were 10 developers present, I'm going to be wondering whether all 10 attendees added value to the meeting. In other words, how do I know you weren't just piling more people into the meeting to rack up the bill?
The way we've addressed this at our work is by only billing for meetings between 2 developers. If you need to get information to 3 or more developers, and you want their feedback, set up an internal Q&A / brainstorming forum. Then you can ask each forum contributor to accurately calculate the amount of time they contributed to the brainstorming session, and bill the client accordingly. Should the client want to scrutinise your bill, you can share the forum conversation with them.
However if you prefer an in-person conversation with 3 or more developers, then you may as well have it over lunch, and send your client the lunch bill (and be frank about why you're doing it this way). Your client will appreciate your resourcefulness as well as your honesty. You get a good meal with some team building thrown in, and they get a good discount.