Why do you want to go to Portland? Portland is not on the coast. If you intend to do the coast of Oregon, you really want to get to Astoria.
Because Portland is the most accessible place to get to from other places, especially from overseas. There is a small airport in Astoria, but I can imagine the logistics/cost of getting twelve people and their bikes there from the UK would be prohibitive. (And they would probably have to take a connecting flight from Portland or Seattle anyway.)
There is a bus that runs from Portland to Astoria, the NW Point. It runs from Portland Union Station and Greyhound Station out to the coast. I'd guess getting twelve bikes on one bus trip would still be an issue.