He means you might have to count him and a guest as coming... probably a lady friend, but it's a big if, because, you should know how women are at your age, so don't count on it.

The plus one thing is usually club slang... like if you're on the guest list, it's your name "plus one".

When i was riding the bus part way to work here in SF a few months back, the bus was always packed with kids going to school. I too was completely dumbfounded by the slang. I SERIOUSLY could not understand half of what they were saying, and i consider myself to be somewhat in touch with youth culture... i'm 25 for gods sake, i shouldn't be feeling that old that fast!