Complex sentence: On the night in question, both groups were "drinking and chatting quite happily", according to a guest who was with his girlfriend in the bar but was suddenly ushered out of the club soon after midnight.
Split into simpler sentences: On the night in question, both groups were "drinking and chatting quite happily". That was according to a guest who was with his girlfriend in the bar but was suddenly ushered out of the club soon after midnight.