If engaged in hand-to-hand fighting then only skirmishing models which are actually touching enemy models can fight. Any models which form part of the unit but which are not touching an enemy model may not fight.
Skirmishers may charge enemy troops exactly like ordinary units. Models are moved individually into contact with their foe. When skirmishers charge in it is usual for the player to form up into a line so that more models can fight. This is fine so long as individual models do not exceed their charge move. Any models unable to reach the enemy may be placed behind the fighting rank loosely to remind players that the unit is skirmishing.
Skirmishers lack a proper formation and are not therefore ideally suited to hand-to-hand fighting. Troops who are not fighting are unable to lend their weight to their friends by pushing forward or stepping in to replace fallen comrades. To represent this no combat bonus is added for rear ranks of skirmishers when it comes to deciding combat results. In effect skirmishers always count as one rank regardless of how many models have piled in behind.