Text (MT): Whenever a creature you control attacks, it gets +1/+0 until end of turn for each other attacking creature that shares a creature type with it.
Each creature is counted once even if it shares more than one creature type. [Morningtide FAQ 2008/01/15]
In a Two-Headed Giant game, only creatures you control trigger the ability and get the bonus, but your teammate's attacking creatures are included in the calculation of those bonuses. [Morningtide FAQ 2008/01/15]
