I ran a Go club at my college for 4 years. We have an activities fair also.
My rule of thumb has always been, take the number of sign ups on your sheet, divide it by 5. That's around the number of new people who will show up to your first few meetings.
From there, how many people continue to come is a product of how enjoyable you make their experience. Just remember that freshmen have lots and lots of options of what to do, and the more enjoyable you make your club, the more people will show up regularly. Here's hoping you get lots of people.
To any smashboarder who goes to this: New players are definitely the lifeblood of our community. Let's make this a really enjoyable event for them, win or lose.
Since we'll have a lot of inexperienced players, format-wise, I wouldn't recommend just a bracket.
I think they would enjoy either of the following two formats:
1. Seeding of ranked or known skilled players into bracket, and putting the remaining people into pools for the other spots.
2. Swiss Round Robin.
They get much more play time this way, instead of getting stuck playing friendlies. Even after they're done, they get to watch the main bracket for more exciting matches.
EDIT: Right.
Obligatory: I'll be going to this, bringing HMW, Phil, Sfat, and Alan.
I'm bringing a cube, 2 melee copies, 2 mem cards, and maybe a dazzle with splitters if anyone wants to record.