Once you have an account, either because you got an invitation or reg is open, most usenet indexers have a free tier that lets you grab a few files per day. The invitation alone is not going to give you a premium account for free.
I do recommend getting on to a couple of good indexers and using the free grabs to see how you like usenet. Personally, I love it, even though it isn't free. YMMV.
Once you have a verdict I am sure a lot of people would enjoy a review post.