Space between players?

[dkmz17]

At a recent game I watched, during a throw in, two offensive players stood in the lane, one behind the other, so close that a defender could not get in between them. A defender did try to move between, but was not allowed to by the referee.

Are the offensive players required to let a defender move between them on the inbounds play or can they stand together as described? If the defender does have the right to get in between the offensive players, where is the rule/authority that grants the player that right?

Thanks,

Dkmz