Delete all the existing vertex groups/armature mods from the shirt and put the character in it's default pose position. Apply the Mirror Modifier if present. Then follow these steps:
Select the shirt in object mode. In the Modifiers panel, select "Add Modifier", select "Data Transfer". In the Data Transfer panel, choose your character and the Source Object, check the "Vertex Data" box, then click the "Vertex Group" button, and then click the "Generate Data Layers" button at the bottom. I usually move this modifier to the top of the stack and once I'm satisfied it works, I'll apply it. Add an Armature modifier to the shirt and select the character's armature in the panel. I usually like to move the Armature mod all the way to the top of the stack and any sub-d mods to the very bottom for best performance.
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