I'd forego the group trip, but definitely connect with someone who's willing to travel w/ you. Get the Triple A guidebooks and maps for WA, OR & CA, as they contain A LOT of really useful info that will help you avoid problems and see the best there is to offer. Spend some time studying the maps and book to get an idea of where you'd like to go and what you'd like to see; there's plenty!
From Seattle, take the ferry to Bainbridge Is. and then head NW to Port Angeles via US 101, a MUCH nicer, if much longer, drive than I-5. Check out Olympic National Park & Forest along the way, and don't try to drive more than 4-5 hrs a day, or you'll be exhausted and miss a lot. Make sure your windshield wipers are in good shape and use gasbuddy.com to avoid being gouged on gas prices, which can be high in the smaller towns, especially in CA.
Stay on 101 as you cross into OR at Astoria, and check out the maritime museum there, just down the road from a KFC! If you want to see Portland, now's the time to cut over on US 30, and back on US 26, rejoining the 101 Cannon Beach. Savor the beautiful OR coast and make a stop in Tillamook to see the dairies and airplane museum nearby. As you work your way down OR's coast, take time to pull off and into the many state parks along the way for some really great views. An overnite in Coos Bay is nice and make sure you head out to the mouth of the bay to see the not-so-pacific Pacific; you'll never complain about high seafood prices again! Head down to Charleston and follow the coast road south before rejoining 101 near Bandon, OR; LOTS of great scenery along this detour. Or, before you get to Coos Bay, head east to Eugene, and then continue into the Cascades Loop to Mt. Bachelor and Bend, and then south to Crater Lake. Either way, get back onto the 5 and check out Ashland, where you can see some great theater if that interests you.
I-5 is the only way to get to Mt. Shasta, and you can spend some time exploring Redding and its environs, especially the dam at Shasta Lake; its tour is a good one. Head west on Hwy. 299 to rejoin 101 at Eureka, which is another cool place to stay before you continue on into the redwoods. Cut west onto Hwy. 1 at Leggett and take it all the way down to Mill Valley, where you'll rejoin the 101 into SF. Or cut over to the 101 via Hwy. 128 just south of Albion, if you want to see the Anderson Valley wine region, or find yourself short of time. I've been doing variations on this route for several years and am still discovering new things on and off the trail. Have a great time!