First of all, objectively present the situation to your team:
Guys, we have been nominated for an innovation award, however only 6 of us, myself included, can attend this event.
Next, tell them when and where the event is taking place, and ask who is interested in attending:
The event is taking place at Restaurant X, on October Y, at Z PM. Who is interested in going?
Finally, if more people want to go than there are tickets, hold a lottery draw.
Here comes the important bit, however: treat the "losers" to some sort of special treat. Maybe lunch out on the day where the rest of you guys are attending the event. Or maybe organize a really nice team event for everyone to attend (however, you should still make sure that the people being left behind get a little something extra).
There's nothing worse than hearing the people who went talking about how great the food/drinks/atmosphere were, when you yourself were excluded. However, give the team their own bonding event to talk about, and that animosity will not build up.