The people that make popular slice-of-life comics like Jose, Yuu, Gemsona, etc have quite a few things that they all do together, both for fun and to boost each-others popularity. Like they did that bucket-thing on April Fool's day, too. The TapProm and all seems to just be more of that. As for whether they all do it by invite or if anyone with a slice-of-life comic can do it, I dunno.
If there's any popular people reading this with inside information, I'd also love to know how this works XD