I'm Lord Jason Silverheart of the former Shire of Oathbreaker Bog in Sherwood. I'm not a part of the Portland group, but I can speak for them.
The Mystic Seas travels as a group to other holdings and Kingdom level events in Vancouver, Salem, Eugene, and Gladstone frequently, but their main park is U.S. Grant Park next to Grant High School, on the corner of NE 36th and Brazee. I checked the schedule and they should be there this coming Saturday the 27th. Most start showing up at 1 in small groups, but the typical attendance should be close to 10 or 15. They usually gather under the covered area, rain or shine. As always play is free, and almost anyone will be willing to lend you a sword if you don't have your own gear.