If your network is to be used by technically more savvy people you could use this approach: On personA site there would be a field which accepts a 'reshare' co

If your network is to be used by technically more savvy people you could use this approach: On personA site there would be a field which accepts a 'reshare' code when the user is logged in. Clicking on the reshare button on personB site would (by default) display a reshare code. This code would contain information such as personB URL, id of the post etc. Pasting the reshare code into personA site would result in a (backend) communication between personA and personB sites to sort stuff out. When using reshare function for the first time on personB site, the site would propose to enter the URL of 'your own' server in order to transform the reshare button functionality. Once personB site knows the url of personA, it would transform the reshare button so it actually constructs an URL in the means of personA/reshare/from/personB/reshare-code. Since the user is presumably logged in (with cookie/local-storage token) to personA, this would redirect him to personA, the backend of personA would eat the reshare-code and could offer a dialog asking for confirmation of a reshare. Once validated it could ping personB site that the resharing was complete.