The set of creatures that gets +1/+0 from an overloaded Teleportal is determined as the spell resolves. Creatures you begin to control later in the turn won't get +1/+0. However, because the second part of Teleportal's effect doesn't change the characteristics of any permanents, the set of creatures that can't be blocked is constantly updated. Any creature you control at the moment blockers are chosen can't be blocked.