Ellen--Did you sign a contract? Would you buy any of the homes that you saw with the other agent? If you can answer no to both questions, then the other agent would not have a right to the commission in Minnesota. (Is Plainsboro in MN?)
If you have signed a contract or if you plan on writing an offer on a house that you saw with an agent, then yes that first agent was the procuring cause to your purchase as they first introduced the home to you. And they could have a right to part or all of the commission for the buyer's side of the transaction. In your scenario, the agent that showed you the home at the open house could have a right to part of the commission since they were the procuring cause.