Read up on collision filtering in the Box2D manual:
Collision filtering allows you to prevent collision between fixtures.
For example, say you make a character that rides a bicycle. You want
the bicycle to collide with the terrain and the character to collide
with the terrain, but you don't want the character to collide with the
bicycle (because they must overlap). Box2D supports such collision
filtering using categories and groups.
Box2D supports 16 collision categories. For each fixture you can
specify which category it belongs to. You also specify what other
categories this fixture can collide with. For example, you could
specify in a multiplayer game that all players don't collide with each
other and monsters don't collide with each other, but players and
monsters should collide
Link to manual here. (Search for Filtering)