"Hit me up" is to contact someone, its used informally, so usually among peers and friends. In this instance, it would be kinda like calling your friends out for not asking you to Brunch.

Its a slang term that in this case means ask. You guys did not ask me to do brunch. The same thing. But him me up sort of implies that someone really is requesting it. Its very slang. More examples, Hey, come hit me up for dinner sometime, means tell me when you want me to treat you for dinner . Hit me up for a movie, Hit me up for a game sometime.. Like that.