We have found that it's easier to get students involved in the team during off-season events. We have a lot of team building outings during the summer and that's where we usually invite our incoming students, that way they get to know the other students and the mentors in a less-stressful environment. We usually do bowling, minigolf, or trips to the beach, stuff like that.
But advertising is also a big part of it, as other people have said.
Another, slightly more corny idea, is a "bring a friend to robotics" day on your team. That's actually how I got involved my freshman year.
I hope this helps!
