A friend who will come to you and mention the plans of an activity they are putting together with other friends, with no intention of actually including you just so they can say they included you in some form and feel like a better friend than they actually are.
Jason: Hey Matt, do you want to come to this party that Jess and some other people are putting together?
Matt: Oh really? Sure! I'd love to go!
Jason: Oh, well wait a minute. I don't think we're gonna have room in the van for you. Maybe next time.
Matt: You're such a Just Because Friend...