[33girl]

Three weeks? As in, July? Huh? You might get more members if you waited until, um, they were there at the school while it was in session.

But anyway, as for how to get upperclassmen:

Talk to upperclassmen in your classes, at the cafeteria, and wherever else you meet them.

Invite them to hang out with your friends. I'm guessing that many of your friends are your brothers.

When you feel you know them well enough, ask them to your official events, and then ask them to join.

Trying to hold "events" under the table can get you in trouble with the school, and is a complete waste of money and time.