Is this a public beta?

No, it isn't officially released for the public.

In other words, can we join this beta forum???

Create a new account on the beta forum to test the forum.

And, will the posts made in the beta version be erased?

Yep, in other words, posts made in beta won't be forwarded to this official forum.

Thanks for the info... I'm going to testdrive the beta version right now

EDIT: our accounts are already on the beta version, apperantly we only need to reset our password