I have read in some guide that finishing group dungeon will give you 1 skill point. I am currently 26 lev. Does it mean that I missed 2 potential skill points?
You missed more than 2 points. Each zone have 1 grup dungeon but after finishing it you can do the other 2 faction dungeons. So you can get 3 skill points per zone. Since you are 26 im assuming you are finishing up the 2nd zone so you have missed 6 points so far.
Good news is that you can go back and do the dungeons. Even if outleveled them you wont get xp for mob lills but you will get credit for finishing and the skill point will be awarded.