Is there a plug-in for WordPress for this? I would like a new visitor to the site to register on the site with the minimum effort (ideally by just entering email address) and stay on the site to view the member only parts of the site. Does a user always have to leave the site to click on an activation email in order to register with a membership site?
there are many plugins which can fulfill your need. http://wordpress.org/extend/plugins/search.php?q=membership&sort=
Yes, it's possible within the wordpress blog system. All you need to do is go to your blog's settings and under the tab General, you'll see the Membership checkbox and you've to set it right. Then, you've to set the New User Default Role as Subscriber. That's it, now your subscribers can register as free member to your site. However, in future if you want to charge your members then I suggest you to grab relevant membership plugins. If you're willing to spend money, then you should go for DAP plugin which is the best with their in-build affiliate management. Next is wishlist for the ease of use. However, if you want something free then go for S2Member. I've worked with many of my clients site by using S2Member. They also providing more options for the pro/paid version of S2Member. If you need any help in setting up things, just PM me and I'll see what can I do for you. Hope this info helps you...
I suggest you try the s2member plugin (free version) first. You can build a membership list quite quickly (for free), then upgrade to the pro version or get WishList later if required. You'll find the s2Member plugin very easy to use. The free version works with PayPal only though. Some of my clients have found this to be an issue.
It helps people to get an easy access by way of links. So most advisors would prefer posting their links to help out people be directed instantly. This is way too helpful for us to have a hassle-free way to gain much need information.