Anyone on the current Roster when a Fight occurs, whether if it’s yours or not, whether if it’s called or not, as long as they have a Disc equipped, the other members will get the effects.
In this case, only 2 of the current Heroes have Discs that grant Shields, Nick [Ju] and Genie [Mi].
We can cross Genie out because for his Shield to trigger, he must be 1) in battle and 2) activating his White Skill, which neither of those is applicable to Genie right now, therefore, Nick must be the one who grants a Shield to the frontmost Ally, which is Sulley. If Mr. Incredible was Ambushed, he wouldn’t get a Shield from Nick.