I would say that putting an open invitation on the internet could be considered "holding out." If it was truly a group of your friends that all had common reason for traveling to XXX for an afternoon, it would fall under the priviledges of your private certificate.
However, logged time can be considered compensation is some cases. Since you posted that you are doing this for the purpose of building time (possibly a form of compensation) and you are possibly holding out by making this thread on the internet, I could see how maybe it could be a grey area.
Just because you aren't charging them enough to make a profit doesn't mean you aren't technically charging them to fly them, hence why someone mentioned the 135 reg. I think by loading a plane with as many people as possible who all chip in while you benefit by timebuilding is borderline and might be construed as you charging them.