What does this sentence mean? "A tray of cocktails floated at us through the twilight, and we sat down at a table with the two girls in yellow and three men, each one introduced to us as Mr. Mumble."

This is a metaphorical way to describe the situation the protagonists were in while expressing in the words used part of the sentimentalism in the situation (how the characters were feeling).

A tray of cocktails floated at us through the twilight. Probably they were at the bar very early at twilight, and the main focus of the sentence is in the cocktails, with the attention of the character. He then expresses they sat on a table with some people they didn't know before, expressing they were just adventuring in the situation, however, the speaker is clearly not that interested for them as he doesn't pay attention to their names (expressed on the phrase "introduced to us as Mr. Mumble.")

Cocktails that a waiter was serving passed by the character the they sat down at a table with two female and three males which were introduced to the character