Sounds like a great event! When we organized an event at Univ of California, San Francisco, called the TOM talks (Transforming Outpatient Medicine) we got some experience engaging providers from community clinics and bringing them in to collaborate.
First, it was an event that that showcased different innovations in primary care at different levels (med students, residents, clinic directors) and so was relevant to them as providers working in clinics with a real drive to innovate and improve care at those sites. We advertised it to have broad appeal, eg. "Health care providers-- doctors, nurses, students -- interested in transforming outpatient practice." We also advertised that it would be a novel format for presenting this kind of work, and so tried to have a "hook".
Second, and most important, I think was that we really pushed on those closest to us to simply get the word out to those colleagues they had that were working in community clinics and to encourage them to advertise. People easily sent emails we asked them to forward and since then those emails have gotten to circles even farther out and every so often we get an email asking about things we're doing and how they can attend which is so gratifying.
Third, in other activities, such as trying to set up specific collaborations for Primary Care Innovations Collaborative, we have contacted those clinics and docs that we know of who are doing amazing work and have a specific request to work with them and get trainees into their sites. That works too!
I think we have specific projects that have a community-participation bent and then these other events where we try to get more creative in getting the word out. Sounds like you guys are already having great events, I hope this is helpful too.