Group Party skill improves drop rates (and slight experience) even with depleted stamina. Take note of the following:
1. Count of professional must be at least 3 different class
2. Class of cats are considered
Po is a Hunter
Maysa is a Mage
Wasabi is a Knight
Goro is a Priest
3. I tried during 10x Mobs (Resistance) and it will mess up with the passive skill, so complete your resistance before combat time runs out.