I've been having better luck getting everyone to drink at the same time recently when I make sure to be in conversation with all of them and then make drinks for everyone. One I'm still having trouble with sometimes though is getting everyone happy at the same time. Sometimes it seems to count it if they are in moods like confident or energized, and sometimes it doesn't. When it doesn't count it, I'm not sure what to do to make everyone happy. It often won't even count my Sim when she has no negative moodlets, and some happy ones, but is in a mood other than happy.
By the way Nutella, you can't get the achievements for getting silver and gold in all of the social events if you don't have the extended content that comes with the game. In my game the only types of social events are birthday party, dinner party, wedding, house party, and a date. From what I hear the deluxe version of the game comes with some other types.