Hi Mark,
Driving between Arinsal and Soldeu will take about 40mins, depending upon the weather conditions and traffic etc, it’s a super easy drive and very well sign posted. From Arinsal you’ll head into La Massana, then onto Andorra la Vella the capital of Andorra. From here you’ll see signs for Encamp/Canillo to the left, this road will take you directly past Encamp, Canillo and onto Soldeu.
If you’re not planning on hiring a car, you could either meet in Andorra la Vella or catch the bus to Soldeu. From Arinsal you’ll catch the L5, which leaves about every 30 minutes starting at 8 until 20:45 and costs €2.75 one way. Upon arrival into Andorra la Vella you’ll then need to catch the L3 bus to Soldeu, which leaves every 20 minutes from 8:40 till 21:40 and cost €4.10 one way.
Some of the reps in Arinsal also sell a ski away day to Grandvalira, this might be a option too.