Once you've gotten all seven, you need to charge up seven
ki bars (by holding Square and X together if you're on PS4 or X and A together if you're on Xbox One), and land a full Square (PS4) or X (Xbox One) combo to summon Shenron.
Teen Gohan, SSGSS Goku and SSGSS Vegeta can all perform level 5 Supers if
your ki bar is charged up that high — perform a level 3 Super and keep the button held down (Gohan's is particularly cool if you're a fan of DBZ.)
You can do this with all three of your characters in a row, but what you might not know is that, if
your ki bar has enough energy, you can tag in a character and have them perform a level 3 Super straight away.
The ki bar adds a level of strategy since you need to conserve and build your ki for a more devastating strike.
There's
a ki bar at the bottom of the screen, which powers special moves, but I wasn't sure exactly how to replenish it, or determine which moves make use of it.