I've been to a no assigned seat reception and whoever was greeting us at the welcome table (member of family/wedding party) let us know then. You can also have a sign so someone isn't stuck with letting everyone know the deal.
What if you have a sign right when guests come into the reception so that is the first thing they see. Or you can have your officiant make an announcement at the end of the ceremony so guests know this right away. This way you don’t have people looking for a seating plan and waiting for someone to tell them there isn’t one.