I’ve been asked several times recently by new members about the legality of riding side by side.  They could not find any mention of it in the DOL motorcycle drivers license pamphlet.
 
While it is safer when riding in a group to ride staggered with the lead MC being to the left or inside of the lane, it is not “illegal” to ride side by side.
 
Reference: http://apps.leg.wa.gov/rcw/
 

Title 46

Motor vehicles

 

46.61

Rules of the road.

(1) All motorcycles are entitled to full use of a lane and no motor vehicle shall be driven in such a manner as to deprive any motorcycle of the full use of a lane. This subsection shall not apply to motorcycles operated two abreast in a single lane.

     (2) The operator of a motorcycle shall not overtake and pass in the same lane occupied by the vehicle being overtaken.

     (3) No person shall operate a motorcycle between lanes of traffic or between adjacent lines or rows of vehicles.

     (4) Motorcycles shall not be operated more than two abreast in a single lane.

     (5) Subsections (2) and (3) of this section shall not apply to police officers in the performance of their official duties.

[1975 c 62 § 46.]