If your guests are, let's say, 'open minded', there's always the party rooms at places like this: http://www.p-aplaza.com/8fnew.htm (which comes out at ￥25,600 a night)
Or these: http://www.p-aplaza.com/10f.htm (which come out at under ￥30,000)
There's plenty more places like this and this might partly explain why some of the 'high end' hotels don't feel the need to allow the hosting of smaller social events

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More seriously now, Claska does allow entertaining (or did allow it...). However, I know someone who held a magazine launch in a room there and they found that the hotel kept shifting the 'rules'. He did organise quite a boisterous affair so I doubt you'd run into the same problems.
The room I'm thinking about is: http://www.claska.com/e/guestroom402.html
Room 402 = 123m2 (plus terrace 41m2) which comes out at 84,000yen
(room 401 is much smaller but also has a large terrace - oryou might just want to go with the idea of a huge terrace. Rooftop 'lounge' here: http://www.claska.com/e/rooftop.html)
- The Claska is VERY picky about people staying overnight, so make sure you make it clear that your guests will be leaving by a certain time.
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Alternatively, if your gathering isn't going to be too noisy, I also know someone who has an Art Gallery near Nippori station.
http://erin5star.jugem.cc/?image=480
Floor plan here: http://higure1715cas.bufsiz.jp/space.html
Again, this would depend on your guests not being too stuffy. I can enquire for you, but there's no guarantee it would be available (may depend on said friend's mood, amongst other things )
I'm guessing it would be cheaper to rent than the gallery spaces at Claska.