Figure out how to get word out among university students in your area. But be wary, most students are fickle as they haven't figured out the whole 'time management' thing and will disappear once classes get serious.
Focus your effort on finding 'core' members. Anyone who has built stuff in the past will likely to show up often and be more reliable. Everyone else are likely dabblers who may show up once a year.
Arrange a meetup no less than once a month, no more than twice a month. Have a plan for each meeting, like 'so and so will talk about computer vision', or build nights.
Seek out members by browsing past posts online - perhaps others in your area are looking.
And meetup.com really helped build up the DC robotics group.