Have a friend who has access to the play test and get them to invite you.

Can be recently added friends, no length of time needed. So someone friendly on here could also add you and get you access. There is an existing thread which you can reach out to, but if by the end of the day no one gets back to you I can add you in ~8hrs.

Typically it takes 1 hour to 1 day for the invite to be visible to you. So someone can send it, after that random amount of time you'll get a Steam notification about it.