Harry Griffen Regional Park 9550 Milden St, La Mesa, CA91942 Home to many events, including La Mesa's free Sundays at Six concert series in June and July, this park is operated by a…More joint powers authority. Agencies overseeing the park north of Grossmont High School are the Helix Water District, Grossmont Union High School District, County of San Diego and the cities of El Cajon and La Mesa. The park accommodates 1,500 people and has parking for 181 cars. The 360-seat amphitheater is available for outdoor weddings, graduation parties, political rallies and other events. The park also offers a horseshoe pit, tot lot and Canine Corners, a volunteer-run dog park.