when pbe will open sign ups

PBE has it's own selecting process, they call new people in to test specific things (example being a Leona player might get access during tank item change testing) Other than that people are generally only admitted if previous PBE testers stop playing and they need to fill in the ranks. Negative behavior in game and similar actions can reduce your chances of being admitted.

For example It took me over a year to get admitted without a single report on my account, and even then I was only let in during the huge influx for the the LCU testing.