I built exactly what you describe at Flights With Friends. But the thing I learned is that group trips are not a democracy. Trips without a dictator who makes the decisions don't actually happen.

Building tools for the dictator is hard because getting the other group members who are not the dictator to actually use any tool is too much work. You are always dependent on the least reliable member of the group.

This is the exact reason that travel cost splitting tools don't work and the dictator just ends up paying for everything and getting the group members to pay them back. If they used a bill splitting tool the flights and hotels would sell out before the slowest member of the group put in their credit card.