Okay, I've found and joined the PSF, and a "Subscribe To ...." bar has appeared, so that's those two issues answered.

That is a privately organised group. The powerseller board link is only visable to sellers who qualify for PS status and you aren't required to sign up for anything. It is automatically enabled when you reach powerseller status (just like this board is open to members without having to join any special group).

I believe there are often get-togethers organised on the Community Spirit board if you are looking to make new friends. If you are looking to meet business contacts then a small business group may be more suitable.