The "Your communities" edition form isn't intuitive in my opinion.
UX-wise, I'd expect it to be added if I click on one of the site's with auto-completion. Why do I need to click on "Add"? If I clicked on the site's via the auto-completion, I am going to add it for sure. There's no reasons, I think, (maybe there are!) that a user would click on the auto-completion but wouldn't want to add the site to the "favorites".
Also, I don't see the need for a "save" button. Again, why wouldn't I want to save this? I've already searched for the communities I wanted, added them. Why would I decide to let it go and "revert" my changed?
I think the "Your communities" edit should add a community when you click on the auto-completion and that the "save" button shouldn't exist, there's no use case for such a button.