Shabu Shabu is great for groups -- if you've never done it you should give it a shot. Kaze is in Chinatown, but there are three shabu shabu spots in the Allston/Brookline area -- I haven't tried them all, but they always seem to have a good crowd.
Depends on the age-group, but Fugakyu has two great function rooms on the 2nd floor where you can enjoy great sushi, laugh and sing and even do some sake bombs if that's your thing.
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