The sign-ups will take place August 22nd so more people can play the game.
I just got this news from Bandai Namco UK Twitter page and the Bandai Namco Facebook.
The original date was set for July 26th but in order for more people to play the closed beta, they rescheduled the sign-up date for August 22nd and will open from September 16th to 18th, which is great because it will allow more feedback from the players.
This does not affect Japan and their sign-ups for the PlayStation 4 closed beta opened July 26th and is currently live.
Sign-ups will be available for PlayStation 4 and Xbox One and there is no news on a PC beta yet. The closed beta will include more stages and features along with 9 playable characters with Future Trunks being one of them.
The full game will release early 2018 for PC, PlayStation 4, and Xbox One.
Source: Bandai Namco Entertainment