nUI already does show which group the player is in to the best of my knowledge... though it may not show on the 25 man panel... I can't recall off the top of my head and am at work, so I can't look to see.
If it's not on the 25 man panel, you could just right click the "Raid25" button to back up to the previous panel and have a peek at which group you're in.
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