Putty and Paint is a neat site, though it's really just an online gallery for figures... so you can probably learn just as much from visiting the site as you would as an actual member. I guess as a member you can leave comments and ask questions, though it can be hit or miss on whether you'll actually get any answers. I'll occasionally ask about how something was done and it isn't all that often I get a response.
As for getting a membership, it used to be that members would periodically get invite codes. I've been checking, but it's been a long time since I've gotten one to give out. As someone else mentioned, perhaps only the moderators are getting them now. In their FAQ it said they're doing this to control how quickly the site grows.
If your goal is to learn from fantasy painters, I think John had some good advice. Massive Voodoo is a great source for tutorials and inspiration. And I would second the Cool Mini or Not recommendation. They've got an impressive gallery, but I think the real reason to go their site is their forum. Check out the Works in Progress section. There are some really great artists there and the forum is where you can see what they're doing, how they're doing it, and ask them questions. It's similar to PlanetFigure, however people tend to create a single work in progress thread and continue to post new projects to it (whereas at PF people usually create a new thread for each new figure). I like it as it makes it easier to follow certain artists. While there are a lot of amazing painters posting in the gallery, there's a smaller number who are active on the forums. In my opinion, some of the best who are frequently on the forums are:
Sproket (David Soper)
http://www.coolminiornot.com/forums/showthread.php?54571-Sproket-s-WIP/page54
Skelettet S (Stefan Johnsson)
http://www.coolminiornot.com/forums/showthread.php?37611-SkelettetS-wip/page191
Megazord_man
http://www.coolminiornot.com/forums...-Mega-s-menagerie-of-miniature-madness/page49
You'll find a lot of threads there with people ranging from novice to pro. I'm
there too and, despite it being a fantasy heavy site, post a lot of my historical figures. I think a lot of people over there would like to see your work as well.