There are a couple of resources that can help you plan a trip down the Columbia River. The RCYC yearbook has a section on cruising the river, pages 59-64. This is a good start.
There are several places down river where you could pick up crew members. There are docks at St. Helens, Rainier, Westport Slough, Cathlamet and Astoria. If your crew were going to leave a car, I would find out about parking. Cathlamet and Astoria do charge for moorage, but it is a reasonable fee. You will find that fees on the river are affordable.
There are many good anchorages. Tongue Point is one, it is just SE of Astoria.
Make sure you have some charts, paper or electronic, for the river.