To be clear, the “share damage” portion of Wilbur’s special alone doesn’t increase or decrease the total damage done, it just evenly divides it. That is, if a group of five heroes had ONLY the “share damage” effect on them, and you hit Hero A for 100 damage, the result would be 20 damage dealt to each of Hero A, B, C, D, and E.
Combined with a +Defense bonus (such as Wilbur applies to your own team), this reduces the damage dealt to the team overall, so each would take less than 20 damage for that same 100-damage hit.
Combined with a -Defense penalty (such as Wilbur applies to the enemy team), this increases the total damage done, so each would take more than 20 damage damage for that same 100-damage hit.
This leads to situations where you can use a “share damage” effect against an enemy team. For example, if you attack a team that has Gunnar on it, and he applies his “share damage and +Defense” - if you then hit the enemy team with Ulmer and apply a -Defense effect, the “share damage” stays in place, but the +Def is replaced with -Def and you can really hit the enemy team for good damage.
I hope that helps to make sense of it. In general, “share damage” is a buff, not an ailment.