I imagine that the John K's invited Robert out of kindness so as to give him a chance to talk to Harriet.. not that they'd make a regular pracitce of it. And I dont see Emma inviting him ot Hartfield except occasionally, prior to H's wedding and perhaps at Christmas... We're told that with Harriet getting engaged to Robert, she was busy with the Martins, and her engagements iwth them, and her friendship with Emma must "sink into a calmer" sort of relationship where they woudl not see each other so frequently....