Training is the only skill that stacks across all characters in the party (not troops). Leader / personal skills get no benefits from any character other than the player / party leader's skill (if you make one of your companions a vassal lord, his/her personal skills will affect his/her own party, but not yours). Party skills can be shared between a companion and the party leader. The total party skill is the highest companion skill plus a bonus based on the leader's skill. For example, Jeremus has surgery 4 and I have surgery 2. My party's surgery skill displays as 4(+1) = 5, which comes from Jeremus's skill plus a bonus for my skill. I get that bonus even for skills where my character is the best in the party. Bonuses are +1 at 2, +2 at 5, +3 at 8, and +4 at 10. So the max skill possible is 14