Anyone can remove themselves from a Party.
I tried to look at your specific scenario: I don't know if it's possible for a group 'owner' to remove an individual at all. When approving a new member, party members get an emailed notice of first name and partial phone number. But after that point, there doesn't even seem to be a way to retrieve a list of your current party members, so I'm not sure there's a way for even the Party originator to remove somebody.
I also don't know for certain what happens if the original Party organizer leaves: my best guess is that all Parties are Zombie Parties, and continue indefinitely on their own, stumbling through the landscape as long as there is at least one existing member. (Any Party member can invite new members to the Party, and any Party member can approve new member requests.