You either get an email from Riot to sign up, or when PBE signup is open again, you can join. Those who get an email just gets to go in earlier.

you can't join until another sign up period starts

During specific sign up periods you can sign up and join the PBE (Unless, of course, you're invited).

No one but Riot knows when those are, and I'm sure if there's another large something coming up that revolves around functionality of the game - like Spectator testing as an example of the invites this time around - more invites will be sent out when there's another big something that needs testing.

Really, I'd say just look at that thread revolving around signups once a day if you're really eager to get into the Beta, other than that you can attempt to put in a support ticket or ask a red, but I'm sure all they'll say is that they'll announce when sign ups are coming, so it might be better to just keep a watchful eye!

I hope this helps, just be vigilant!

Alright guys, I got my question sorta answered haha. One last question, When PBE sign up period is open how long does it normally last? Is an hour? A day? A week?