@Miriam164 First of all, any guest who wants to invite other friends or family over needs to ask the host if this is acceptable. It just seems like common courtesy to me, regardless of whether "no parties" is stated or not. So if they didn't ask, then no, I wouldn't allow it. It's totally presumptuous.
Many hosts don't allow any unregistered guests on the property at all, period. Some don't mind, depending on whether the guest has requested permission, and on what type of gathering it is.
I myself wouldn't mind a guest who had asked first, having a friend over for a drink or to share a meal. I don't consider an small adult dinner party to be a "party" like some rager with loud music, drunken behavior and a cast of thousands. But if it's a big family gathering, with kids running around, lots of noise, and so on, no way, that's a party for sure.
To be considered is, if they have people over, all those people are using your amenities- water, toilet paper, towels, soap, etc. For free. And likely creating a whole lot of garbage. Nor will you have any support from Airbnb on a claim for damages if one of their "guests" breaks something, plugs up the toilet, etc.