This is not a new idea. Hola has been around for years. Trying to protect users through access control is foolish. It's like running a Tor exit from home.
I guess it's a bit different for two reasons: 1) Here users are using the bandwidth and it's not resold to companies like Hola does is with https://luminati.io . At least for now. 2) They whitelist domains, so they could only whitelist example.com and you know it's not like Tor where everything goes or Hola where someone is web scraping things through your IP.
But the very idea of sharing my uplink is anathema. Maybe if everyone curated their own whitelists. But once people rely on whitelists from "trusted" peers, all bets are off.
A safer alternative would have users sharing access to each others VPN service connections. That would at least insulate users somewhat from malicious/illegal traffic routed through them.
Indeed, I routinely route traffic through nested chains of 3-5 VPN services. A common criticism is the cost of multiple accounts. And I typically have even more accounts at any given time, for variety.
But if a bunch of people pooled access to their VPN services, or to VPNs that they ran privately on anonymously leased VPS, each one could have a much larger variety of VPN paths and exit IPs. And you could multiplex and split traffic through the VPN network, to increase anonymity. Or aggregate links, using MPTCP, to increase throughput. And you could even implement something like Tor's process of switching circuits every 10 minutes.
I bet that I could implement a simple version of that with routing tables and iptables rules. And some shell scripts. Perhaps with network namespaces, for a little more security. Even Docker, maybe.
But not just sharing ISP uplinks. That will end in tears.