Most definatley agree with Tony's suggesting in jumping into any aeroplane you can with already qualified pilots...
When I was learning to fly I had the pleasure in flying in the right hand seat of many types from the defacto Warrior and Cessna's through my current type (a Beagle Pup) to a good few unlogged hours in the ultimate of piston twins... a Cessna 340 (a pressurized twin, felt like a mini-airliner).
Within no time at all you'll notice that us bunch are a sociable lot and will always be glad of having someone along for the ride... my thinking is that there is no point in having a spare seat when someone on the ground would like to fly...
All the best.