Just slide the canopy back, flip one of the front seats forward and let the rear passenger step in. Its actually easier in my opinion to get in the back seat of the sliding canopy than a single door low wing in the same catagory.
Lots of positives on the Tiger (we could start another thread)
But the the draw backs related specifically to your post on the Tiger back seat are:
1) When you slide the canopy all the way back it covers up the handle for passengers to use while climbing on the wing. So leave it forward until every one is on the wing (step area) before sliding it all the way back.
2) If you are putting 2 in the back seat then have the front passengers on the front of the wing before the two in the back seat fully load, otherwise the plane will tip back from aft CG when you put weight on the wing step to get on.
3) Rear seat passengers taller than 5'-9" will bump their head on the slope of the roof in turbulence
4) while the rear seat folding down is AWESOME for 2 people and lots of baggage (and skis) . It stinks for 3 and long baggage (like skis) cause it doesn't split.
Thats about it regarding the back seat, but dont take my post the wrong way. The tiger is a great bargain and fun plane to fly with many positives, and a few negatives (like any plane)