Our large group had an absolutely wonderful stay at this AirBnB. It was extremely spacious and clean and is located in a great location for local food. Taxis are plentiful and cheap to get anywhere you'd like, though the walk to downtown is only 25 min or so.
Carole, the host, was incredibly helpful and responsive to my questions and went out of her way to help set up an in-house chef for a cooking class (highly recommend!!) and in giving us the inside scoop for dinner locations and tourist attractions. She even called a local restaurant for us to make a dinner reservation!
If you are looking for a place to stay in Athens for a big gathering, look no further!