Okay, so here are two ways you can recruit team members on your FIRST robotics team. Comment below if you have some more ideas!
This is a simple event that can add members to your team. Organize a small, simple picnic outside the school or a home, create colorful engaging signs that advertise the event, showcase the recent robot you built or other toys the team has created, and then invite students from the school or other contacts your team has. Make sure you advertise the event with creative and eye catching photos of the robot and the organization (FIRST robotics). Students want to see how awesome it can be, so try to appeal to them as much as possible; don't just talk about how great it will be for their future, they will figure that out later.
Have a booth at fall festivals or other events
The idea here is to cater to the younger generation. Go wherever they go, do whatever they do. Festivals or events of that nature are certainly a great place where some potential team members might be, even if it does cost a little bit. Make sure you have exciting and engaging content at the booth to share with the people who walk by. A good example would be big bright signs that show what FIRST Robotics is, or a preview of the robot.
There are of course other ways, so please share your ideas!