I've been to country meetings with democrats in swing districts in CA, PA and CO all in the last month. At every meeting turn out was at least 500% of what it had been in the previous years. CO-6 meeting I was at most recently had a 2000% increase in engagement.
That being said I saw you link to swingleft.org which is run by people who to their own admission have no clue what they are doing. I would encourage people to get involved with the actual Democratic Party in the districts where we can make a difference. Institutional knowledge is too important to toss away in the 2018 election by a bunch of energized progressives Looking toward a new fad group of unknown and unproven qualities for guidance.
Liberal of all stripes especially those who have been disenfranchised in the past need to show up and join the party at the town, county and district level. You would be amazed by the impact that can be had. When I was in CA I was proud to watch an energized and organized group of Sanders supporters put themselves in the position where they will have the loudest voice in the next CA party platform.
I have nothing against swingleft in principle (or I wouldn't if they had someone from Wellstone or MoveOn at the helm) but the fact is by engaging with the Democratic Party we can help make it reflect our concerns while also taking advantage of, data, resources and party activist with real experience. The only way to make our party better and stronger is to show up.