Accommodations in Snohomish are limited to a few bed and breakfasts, which are probably already booked. Liz and Ian recommend looking for hotels in Seattle or Everett, and they will help arrange carpools to Snohomish for the wedding.
Link light rail runs quickly from Seatac airport to downtown Seattle for about $3. Buses run frequently all around Seattle for about $2.50-$2.75, depending on time of day, in quarters. Certain buses run north to Snohomish County and Everett for up to $3.25. More information can be found on the King County Metro Transit website: http://metro.kingcounty.gov
Unfortunately, public transportation options to downtown Snohomish are limited. Liz and Ian can put guests in contact with each other to arrange carpools to Snohomish from Seattle, Everett, Mukilteo, or many other locations.
Anyone traveling by air should fly into Seatac airport (SEA), which is about 30 minutes south of Seattle and about an hour from Snohomish.
For those traveling from the Philadelphia area, US Airways and Alaskan Airways offer direct flights.
For those traveling from Maine, Ian and Liz have found the most price and time efficient method is to take Cape Air into Boston Logan and a direct flight from there to Seatac. Buses run from Portland to Boston as an alternative. Flights from Seattle to Portland typically involve several layovers and high costs.
For those traveling from Australia, best wishes on the long flight.