I'd say Lovino has the most friends over, due to him having the most friends out of all of us- at least, the most friends in a consistent group, if you don't count the mutual friends the boys share.
Feliciano's friends are a fairly mellow bunch- a bit loud at times, but it's nothing drastic. There is a bit of shouting (particularly at Feliciano) from the blonde one, but I think they handle the situations just fine. Having them over is alright.
Lovino's friends are an interesting group- mostly made up of mutual friends between him and Feliciano, childhood friends, and pretty girls (my, my!). They're kind, and all get along fairly well. Having them over is always nice!
Giovanni's friends are all a bit of a handful, since they're all fairly young, but I think that the girl can handle the boys fairly well. She puts up with a lot, I respect that. Things get a bit messy when they're over, someone usually ends up hurt.
And my friends are either love interests or out to kill me! Having them over is a real party.