The person that goes along with you to events, or just accompanies you through something like a night out or the rest of your life.
From invitations and guest lists, ie. Gary Applebottom +1.
"sorry, couples only - put an ad on craigslist"
''I'm surprised he's not already a pilot. He's got the drinking part down.''
+1 refers to the fact that it adds 1 more person to their friends list.
Guy 2 - Most of them are +1s, out of the 300 friends he has he probably only knows 80 of them.
Guy - "(insert name here) wants to add me? I don't really like him but what the hell +1 I suppose."