A filler friend is the one who is always walking behind the group when there's no room left or will often times be forgotten to be invited to events or gatherings. Usually ends up the designated driver when actually invited.
Jeff: "Hey man, that movie was badass!"
Eric: "Yeah dude, it was awesome!"
Tyler: "Oh.. you guys went to the movies last night?.." I'm so tired of being a filler friend..
An acquaintance that you invite to important functions such as weddings and baby showers, but wouldn't normally hang out with otherwise. Like a carnation is a filler flower in a bouquet, the filler friend gives out the illusion that you have more friends than you actually do.
Since my fiance invited all of his hockey team to our wedding, I sent out an invitation to all of my filler friends so that I won't look like a loser with no friends at my wedding.