Barry Allen to Recruit the DC TV Superheroes for Upcoming Crossover

It looks like Barry Allen may be the reason behind the upcoming DC TV 4 show crossover. The Flash producer Todd Helbing has revealed that it will be Barry recruiting the other three shows’ superheroes to take on the Dominators in the upcoming crossover.
Helbing revealed to Entertainment Weekly that The Flash’s portion of the crossover is the “…first act of a three-act movie. It’s sort of where Barry learns of the Dominators and has to recruit all of the people.”
This would explain why it was mentioned at San Diego Comic Con that the crossover would start with a Flash episode and not a Supergirl one. Though the question still remains, how Barry gets Supergirl to his Earth? Does he go to her Earth like he did in their crossover and recruit or do the Dominators have something to do with her being there?
Also, this does not seem like a 4 show crossover. When I first heard they were doing a crossover with all four shows I thought it would be a four episode crossover. From the sound of it, it seems more like a three show crossover with the addition of Supergirl. I don’t think her cast has been filming the crossover and it doesn’t sound like they will be in the episodes. I hope I am wrong and they some how make it into them, but right now it looks like they aren’t.
What do you guys think?