Just make sure there are some cute guys on the team.
j/k
You could contact local clubs/organizations for girls (such as the Girl Scouts), and try to get some of them involved. You can even set up some kind of presentation for them to show them what robotics is. Girl Scouts are really into learning about career opportunities, and would probably love to get more girls into technological fields, so I bet you would get some co-operation there.
Like santosh said, they are also more likely to come in groups. Girls can be shy about joining a guy-dominant group like robotics, but if there are other girls they have more of a chance.