Right after Peggy has picked up her coat and scarf to leave, it cuts to a shot of Debra and Peggy can be seen behind her with her coat already on.
When Marie goes to the kitchen to make party snacks, she passes the couch right in front of Peter and Peggy. It then cuts to a close-up of the couch and Marie is not visible, and not enough time passed for her to walk by.
Debra's friends from her book club were supposed to come to the party to meet Peter - but they never showed up.