These are the agencies listed in the Multnomah County SO's press release:
Multnomah County Sherriff's Office / SAR
SearchOne Canine Inc.
Portland Police Bureau
Major Crimes Team (PPB, Gresham Police, Fairview Police, OSP, and the FBI)
Portland Mountain Rescue
Yamhill County Canine
Washington County Sherriff's Office / SAR
Yamhill County Sherriff's Office
Clark County Sherriff's Office
Pacific NW Search and Rescue
Oregon National Guard
What they don't list is that at one point yesterday (or the day day before?) they had Tualatin Valley Fire & Rescue's Dive Rescue team up in the area, because there is a large pond across the street from the school. Whether TVF&R is still helping with the search is unknown.
As for the frequencies...unfortunately, I haven't been following the incident because of work.