This happened to me a couple of times already! During a party one of my sims is hosting, I see party guests stuck at the left & right side of the street. They're just standing there. But, when I click on them to interact, they go to the green grass area in front of the house. But, they don't act the way they're supposed to like when they roam around the house, talk to one another, etc.
I observed that this happens when one or all of my sims are completing other events like Llamazoom tasks or just doing hobby, career or relationship events while there is a party being hosted in my sim's house.
I'll try to post a pic when I host my next party.