A battle standard bearer can join a unit of troops in the same way as any other character. If he is in the front rank of a unit that is fighting in close combat, or if he is anyway fighting the enemy (he could have moved to fight enemies engaging his unit from the rear, for example), his side receives an extra +1 combat bonus when working out combat results.
À unit can benefit from both its unit standard bearer and the battle standard bearer bonuses, therefore receiving a +2 combat result bonus.