Advertising a symposium
Some suggestions for improving participation:


Recruit two to three faculty members to serve on a panel to judge presentations. The top three (or four or five) presentations receive prizes. Include this on the flyer.
Every participant gets a certificate of participation.
Announce a recognized name as a keynote speaker, if possible. Not a big deal, if it's not possible.


Then, mail the flyer to the graduate program directors (of the relevant departments) at the nearby universities and ask them to encourage their students to participate in the symposium. Also contact the graduate student organizations at these universities.