Jill is allowed to invite people into her place of dwelling, despite Tybalt's repudiation, and at the end of the conversation he backed down. He later confirms that he acknowledges Jill's open invitation:
Tybalt: "And I'm aware, you said he's welcome here, but I think you should reconsider."
Under the circumstances of a guest entering a room and not stealing or damage anything, I'd be interested if a case of trespass was every successfully prosecuted.
Of course they are following the MC. The entire "earn their own keep" approach has been embraced by the entire frat. Also the cleanup which the MC is 100% coordinating is being embraced by everyone. Even Tommy pitched in eventually after the MC knocked some sense in him.