I worked for a startup 4 years ago that started off with this very premise: enabling users to buy tickets to events in real time. The company very quickly changed focus to just being a ticket search engine, due to the assumption that the money was in helping people find the best price not shopping with their friends.

Don't let that dissuade you though. You might be on to something. Planning trips with a group is a painful experience considering conflicting schedules & budgets. Once users clue you in to their intended destination and budget you could find plenty of ways to monetize this.