How do you persuade more than one person at a time? You need to acknowledge group decisions don’t get made in group settings.
Think about that: It’s counterintuitive but inescapably true. Groups hear and discuss, sometimes debate and argue, but they seldom decide as a unit. Rarely will you find a single decision maker. Rather, multiple decision makers — often including but not limited to the budget manager, a hierarchical leader and an informal leader — are involved in the final decision.
Thus, you need to appeal to fiscal prudence, leadership responsibility, charisma or all of the above. Group meetings must be augmented by one‐on‐one meetings to gain support and woo true decision makers. Consider yourself a congressional lobbyist, but one with scruples and a good cause.
You don’t need unanimity or an overwhelming mandate to generate group agreement; you need critical mass. Consensus is something everyone can live with, not something everyone would die for. With that in mind, focus on the pragmatism of the numbers. That means “being right” in your own mind isn’t good enough.