Many units in Warhammer 40,000 contain models with different levels of Weapon Skill. Attacks against a unit are resolved using the Weapon Skill of the majority type in the unit. For example, if a mob of twenty Grots (WS2) is led by an Ork Slaver (WS4), while the Grots are in the majority then Attacks against the unit are resolved using their WS of 2 - however skilled the Slaver is, they can't prevent the Grots getting hit in the first place. If there is no majority Weapon Skill type, use the lowest Weapon Skill in the unit.
When models attack, calculate their own To Hit numbers based on their own Weapon Skill. If the Grots and Slaver hit back at their enemies, the Grots' To Hit number will be based on a comparison of WS2 with the enemy WS, the Slaver's on a comparison of WS4 with the enemy WS.