Sounds like an EiT event! Same criteria. Think you want to try and reserve somewhere? Where do you want to take them - Taksim/Tünel area?
Our last party was at
Venta Del Toro in Galata. Cheap place for beers but wine/cocktails not up to much. There's now a YTL10 charge for the Cuban band per person. Friday is quieter than Saturday so you could reserve downstairs.
In Tünel you could go to
Tocc which is the old Taps/Peradox and is empty so more likely you can get a deal on drinks.
Or the bar opposite House Cafe - does it even have a name

Empty and has live music sometimes.
Moving up the street, we had a party in
Nar Pera, which is down the street with Adidas at the top. Small venue, could fit 50 per floor without chairs. Can get smoky. You'd be able to reserve the place. Next door is
Carnival, quiet more of a restaurant vibe.
You could try your luck at any of the
French Street bars as they always need business.
Nevizade is the ideal place if you can find somewhere to accommodate you. We did
Nevi two years ago and it's got a few floors so you could take one over - not too dark/dirty/studenty/bad beer like other bars around there.
Balo Sokak: You could do
Balans and reserve upstairs on the first floor but just make sure you get what you want. Can't remember if
Au Pera is still operating under the same name but you could reserve a floor there and get a deal on drinks.
Loads of bars on
Imnan Adnan Sokak. Few around the back streets but the crowd can be a bit iffy.
Cafe Krepen next to Balo Sokak was suggested to me for an EiT party as it has a big garden. It's replaced a very old bar.
How about
Me Gusta next to Garanti in Taksim Square? You could get your guys a reservation downstairs. Cheap, got those beer dispenser things and the crowd is really mixed, especially after work. There's an equivalent bar across the square that was used for cruising Beer Manca or something...
Perhaps you can have a word with venues that get busy later around 10pm to see if you can go there early - e.g. Mono
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You could do what we're doing for the EiT Feb party and go to Cihangir - we've looked at
Susam and
Noa (above former Leyla) for parties...Noa has a great view but toilets are 2 flights down. Susam is quietish compared to the other bars.
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If you want other areas of town I can have a think