We were a student group and stayed there for 9 nights. But we had problems as the flat is clearly not designed for 16 people. Just that it lacks a large enough table, 16 glasses, 16 chairs etc. makes it hard for everyone to come together and chill, or eat breakfast. The common area is quite small, and does not suit a bigger group.
One of the toilets was broken and spilled some of the water you wanted to flush out on the bathroom floor.
The rooms are many and small but quite nice, although two of the bedrooms come without windows. Although some of the have balconies.
The location is fine, and taxis are really cheap in Portugal, also you have a supermarket really close to the apartment. Also the price is better than most of the bigger apartments in Lisbon.
However, I wouldn't really recommend this to a large group.