Following is the Bender’s House Rule for Flanking:
When two allies are able to make melee attacks from opposite sides of an enemy, they will be considered to be Flanking the enemy. By getting into this Flanking position, both allies will get a +2 to their attack rolls.
Once two allies have achieved the flanking position, every additional ally that makes a melee attack on the enemy will add +1 to the attack rolls of all the allies. For example, if a third ally joined the two allies in the above example, all of the allies would gain +3 to their attack rolls.
At least two of the allies must be directly across from each other with the enemy in the middle in order to invoke the Flanking rule. Having multiple allies attacking a defender does not automatically trigger the Flanking rule. At least two of them must be directly opposite each other.