If you control Sphere of Safety, your opponents can choose not to attack with a creature with an ability that says it attacks if able.

In a Two-Headed Giant game, if one player controls Sphere of Safety, creatures can't attack that player's team or a planeswalker that player controls unless their controller pays for each of those creatures he or she controls. Creatures can attack planeswalkers controlled by that player's teammate without having to pay this cost.