Talk to the girls on the team currently and find out what made them want to join. I know there are guys on the team that have at least ONE female friend. Drag them to a meeting. Just make sure you don't ONLY talk about video games. If building is not their forte or something they're interested in make sure you show them all the OTHER aspects of a FIRST team. I'd be more than willing to share my experiences on a FIRST team that started with just ONE girl (me), and now has 8 I believe.
