The machine has to be built for it
RCW 46.61.610 says the operator shall ride only upon the permanent and regular seat attached to the motorcycle, and shall not carry another person unless the motorcycle is designed to carry more than one person.
Where the passenger sits is specified too: either a permanent seat designed for two, or an additional seat firmly attached at the rear or the side of the operator. A sissy bar, a luggage rack, or a friend perched on the tank do not qualify.
Footrests are required, not optional
The motorcycle must have foot pegs for each passenger it is designed to carry, or an additional bucket seat with a seat belt meeting the federal standard under 49 C.F.R. Part 571.
This is the requirement most often missed on a bike that came from the factory as a solo machine, or one where a previous owner removed the rear pegs for looks. If the pegs are gone, the bike is not legally set up to carry anyone.
Your passenger needs a helmet
The helmet statute, RCW 46.37.530, applies to any person who operates or rides upon a motorcycle. Your passenger needs a helmet meeting the same federal certification standard you do. Details are in the helmet law guide.
If you are on a permit, the answer is no
A Washington motorcycle instruction permit carries two hard restrictions: the holder may not carry passengers, and may not operate a motorcycle during the hours of darkness. The DOL charges $35 for a first permit and $15 to renew, each valid 180 days, and you may hold at most two within any five-year period.
So the honest answer to when you can take your partner for a ride is: after you are endorsed. Which is a decent reason not to let the permit clock run out. See do I need a permit first.
Children: five is the floor, and reaching the pegs is the real test
Washington sets a hard minimum. RCW 46.37.530 makes it unlawful "to transport a child under the age of five on a motorcycle or motor-driven cycle." There is no exception for a short trip or a parking lot.
Five is the legal floor, not the standard. The DOL puts the practical test plainly: your child can be a passenger if they are at least five and able to reach the foot pegs. A child whose feet dangle cannot brace against braking, cannot stay planted at a stop, and is a hazard to both of you. Age is the law. Reaching the pegs is the actual requirement.
Your child also needs a helmet meeting the same federal certification standard you do, in a size that genuinely fits. An adult helmet on a child is not compliance, it is a loose weight on a small neck.
What changes when you ride two-up
Carrying a passenger changes the motorcycle. Braking distance goes up, the front gets light under acceleration, and low-speed balance is a different skill entirely. Riders who have only ever ridden solo are usually surprised the first time. Practice somewhere empty before you take anyone into traffic, and brief them: hold on, look over my shoulder in the direction of the turn, and do not move on their own at a stop.
Two-up technique is one of the things our advanced course works on with riders who already have the endorsement.