I don't know of anyone who has joined, but it will be interesting to see if this model succeeds. Parent Booster USA was formed last year by Sandra Nau, a non-profit lawyer who used to be an article contributor to PTO Today magazine and sometimes spoke at the conferences. I think the site is just really getting off the ground now.
Parent Booster is set up as an non-profit itself and serves as an umbrella organization for others. As I understand it, you join it, something like joining an association, and their 501(c)(3) status covers you without filing the paperwork yourself as long as you pay the annual fee to sustain membership.
They provide resources and an online discussion forum - but only for members. Nothing is available for Guests as it is on the PTO Today website.
It doesn't seem like Parent Boosters USA has any real authority over its members (the way the National PTA does over its chapters). So I'm not sure how they would control if a member follows appropriate rules for maintaining 501(c)(3) status.
Years ago, Sandy, an IRS Rep, and I had very lively discussion about PTOs and non-profit status and what was necessary for a PTO to do/not do. It was very interesting to hear their opposing viewpoints.
